diff --git a/packages_rs/nextclade/src/analyze/divergence.rs b/packages_rs/nextclade/src/analyze/divergence.rs
index 381a509ab..bf389863d 100644
--- a/packages_rs/nextclade/src/analyze/divergence.rs
+++ b/packages_rs/nextclade/src/analyze/divergence.rs
@@ -1,4 +1,6 @@
 use crate::analyze::nuc_sub::NucSub;
+use crate::coord::range::NucRefGlobalRange;
+use crate::tree::params::TreeBuilderParams;
 use crate::tree::tree::DivergenceUnits;
 
 /// Calculate number of nuc muts, only considering ACGT characters
@@ -24,3 +26,17 @@ pub fn calculate_branch_length(
 
   this_div
 }
+
+/// Calculate nuc mut score
+pub fn score_nuc_muts(nuc_muts: &[NucSub], masked_ranges: &[NucRefGlobalRange], params: &TreeBuilderParams) -> f64 {
+  // Only consider ACGT characters
+  let nuc_muts = nuc_muts.iter().filter(|m| m.ref_nuc.is_acgt() && m.qry_nuc.is_acgt());
+
+  // Split away masked mutations
+  let (masked_muts, muts): (Vec<_>, Vec<_>) =
+    nuc_muts.partition(|m| masked_ranges.iter().any(|range| range.contains(m.pos)));
+
+  let n_muts = muts.len() as f64;
+  let n_masked_muts = masked_muts.len() as f64;
+  n_muts + n_masked_muts * params.masked_muts_weight
+}
diff --git a/packages_rs/nextclade/src/run/nextclade_wasm.rs b/packages_rs/nextclade/src/run/nextclade_wasm.rs
index a8fcbbc77..0873e8537 100644
--- a/packages_rs/nextclade/src/run/nextclade_wasm.rs
+++ b/packages_rs/nextclade/src/run/nextclade_wasm.rs
@@ -257,7 +257,7 @@ impl Nextclade {
 
   pub fn get_output_trees(&mut self, results: Vec<NextcladeOutputs>) -> Result<Option<OutputTrees>, Report> {
     if let Some(graph) = &mut self.graph {
-      graph_attach_new_nodes_in_place(graph, results, self.ref_seq.len(), &self.params.tree_builder)?;
+      graph_attach_new_nodes_in_place(graph, results, self.ref_seq.len(),  &self.params.tree_builder)?;
       let auspice = convert_graph_to_auspice_tree(graph)?;
       let nwk = convert_graph_to_nwk_string(graph)?;
       Ok(Some(OutputTrees { auspice, nwk }))
diff --git a/packages_rs/nextclade/src/tree/params.rs b/packages_rs/nextclade/src/tree/params.rs
index 4171ee034..d751d2324 100644
--- a/packages_rs/nextclade/src/tree/params.rs
+++ b/packages_rs/nextclade/src/tree/params.rs
@@ -14,6 +14,9 @@ pub struct TreeBuilderParams {
   #[clap(long)]
   #[clap(num_args=0..=1, default_missing_value = "true")]
   pub without_greedy_tree_builder: bool,
+
+  #[clap(long)]
+  pub masked_muts_weight: f64,
 }
 
 #[allow(clippy::derivable_impls)]
@@ -21,6 +24,7 @@ impl Default for TreeBuilderParams {
   fn default() -> Self {
     Self {
       without_greedy_tree_builder: false,
+      masked_muts_weight: 0.05,
     }
   }
 }
diff --git a/packages_rs/nextclade/src/tree/split_muts.rs b/packages_rs/nextclade/src/tree/split_muts.rs
index 50493824e..3057150cc 100644
--- a/packages_rs/nextclade/src/tree/split_muts.rs
+++ b/packages_rs/nextclade/src/tree/split_muts.rs
@@ -244,9 +244,9 @@ where
 pub fn difference_of_muts(left: &BranchMutations, right: &BranchMutations) -> Result<BranchMutations, Report> {
   Ok(BranchMutations {
     nuc_muts: difference(&left.nuc_muts, &right.nuc_muts)
-      .wrap_err("When calculating union of private nucleotide substitutions")?,
+      .wrap_err("When calculating difference of private nucleotide substitutions")?,
     aa_muts: difference_of_aa_muts(&left.aa_muts, &right.aa_muts)
-      .wrap_err("When calculating union of private aminoacid mutations")?,
+      .wrap_err("When calculating difference of private aminoacid mutations")?,
   })
 }
 
diff --git a/packages_rs/nextclade/src/tree/tree_builder.rs b/packages_rs/nextclade/src/tree/tree_builder.rs
index 5dc31e638..6955ca65a 100644
--- a/packages_rs/nextclade/src/tree/tree_builder.rs
+++ b/packages_rs/nextclade/src/tree/tree_builder.rs
@@ -1,11 +1,11 @@
 use crate::analyze::aa_del::AaDel;
 use crate::analyze::aa_sub::AaSub;
-use crate::analyze::divergence::{calculate_branch_length, count_nuc_muts};
+use crate::analyze::divergence::{calculate_branch_length, count_nuc_muts, score_nuc_muts};
 use crate::analyze::find_private_nuc_mutations::BranchMutations;
 use crate::analyze::nuc_del::NucDel;
 use crate::analyze::nuc_sub::NucSub;
-use crate::graph::node::GraphNodeKey;
-use crate::make_internal_report;
+use crate::coord::range::NucRefGlobalRange;
+use crate::graph::node::{GraphNodeKey, Node};
 use crate::tree::params::TreeBuilderParams;
 use crate::tree::split_muts::{difference_of_muts, split_muts, union_of_muts, SplitMutsResult};
 use crate::tree::tree::{AuspiceGraph, AuspiceGraphEdgePayload, AuspiceGraphNodePayload, TreeBranchAttrsLabels};
@@ -87,7 +87,7 @@ pub fn graph_attach_new_node_in_place(
   } else {
     // for the attachment on the reference tree ('result') fine tune the position
     // on the updated graph to minimize the number of private mutations
-    finetune_nearest_node(graph, result.nearest_node_id, &mutations_seq)?
+    finetune_nearest_node(graph, result.nearest_node_id, &mutations_seq, params)?
   };
 
   // add the new node at the fine tuned position while accounting for shared mutations
@@ -97,109 +97,143 @@ pub fn graph_attach_new_node_in_place(
   Ok(())
 }
 
+/// Moves the new sequences, defined by its set of private mutations
+/// along the tree starting at the `nearest_node`. As the new sequence is moved, the
+/// private mutations are updated. This is repeated until the number of private mutations (nuc)
+/// can not be reduced further by moving the node. At the end of the loop, the nearest node
+/// is either the closest possible point, or this closest point is along the branch leading
+/// to the nearest_node.
 pub fn finetune_nearest_node(
   graph: &AuspiceGraph,
   nearest_node_key: GraphNodeKey,
   seq_private_mutations: &BranchMutations,
+  params: &TreeBuilderParams,
 ) -> Result<(GraphNodeKey, BranchMutations), Report> {
-  let mut current_best_node = graph.get_node(nearest_node_key)?;
+  let masked_ranges = graph.data.meta.placement_mask_ranges();
+  let mut best_node = graph.get_node(nearest_node_key)?;
   let mut private_mutations = seq_private_mutations.clone();
 
-  // the following loop moves the new sequences, defined by its set of private mutations
-  // along the tree starting at the `nearest_node`. As the new sequence is moved, the
-  // private mutations are updated. This is repeated until the number of private mutations (nuc)
-  // can not be reduced further by moving the node. At the end of the loop, the nearest node
-  // is either the closest possible point, or this closest point is along the branch leading
-  // to the nearest_node.
   loop {
-    // in each iteration, check how many mutation are shared with the branch leading to
-    // the current_best_node or any of its children (loop further below).
-    let mut best_node = current_best_node;
-    let (mut best_split_result, mut n_shared_muts) = if current_best_node.is_root() {
-      // don't include node if node is root as we don't attach nodes above the root
-      let best_split_result = SplitMutsResult {
-        left: private_mutations.clone(),
-        right: BranchMutations::default(),
-        shared: BranchMutations::default(),
-      };
-      (best_split_result, 0)
-    } else {
-      let best_split_result = split_muts(
-        &current_best_node.payload().tmp.private_mutations.invert(),
-        &private_mutations,
-      )
-      .wrap_err_with(|| {
+    // Check how many mutations are shared with the branch leading to the current_best_node or any of its children
+    let (candidate_node, candidate_split, shared_muts_score) =
+      find_shared_muts(graph, best_node, &private_mutations, masked_ranges, params).wrap_err_with(|| {
         format!(
-          "When splitting mutations between query sequence and the nearest node '{}'",
-          current_best_node.payload().name
+          "When calculating shared mutations against the current best node '{}'",
+          best_node.payload().name
         )
       })?;
-      let n_shared_muts = count_nuc_muts(&best_split_result.shared.nuc_muts);
-      (best_split_result, n_shared_muts)
-    };
 
-    // check all child nodes for shared mutations
-    for child in graph.iter_children_of(current_best_node) {
-      let tmp_split_result =
-        split_muts(&child.payload().tmp.private_mutations, &private_mutations).wrap_err_with(|| {
-          format!(
-            "When splitting mutations between query sequence and the child node '{}'",
-            child.payload().name
-          )
-        })?;
-      let tmp_n_shared_muts = count_nuc_muts(&tmp_split_result.shared.nuc_muts);
-      if tmp_n_shared_muts > n_shared_muts {
-        n_shared_muts = tmp_n_shared_muts;
-        best_split_result = tmp_split_result;
-        best_node = child;
-      }
+    // Check if the new candidate node is better than the current best
+    let left_muts_score = score_nuc_muts(&candidate_split.left.nuc_muts, masked_ranges, params);
+    match find_better_node_maybe(graph, best_node, candidate_node, shared_muts_score, left_muts_score) {
+      None => break,
+      Some(better_node) => best_node = better_node,
     }
 
-    // if shared mutations are found, the current_best_node is updated
-    if n_shared_muts > 0 {
-      if best_node.key() == current_best_node.key() && best_split_result.left.nuc_muts.is_empty() {
-        // All mutations from the parent to the node are shared with private mutations. Move up to the parent.
-        // FIXME: what if there's no parent?
-        current_best_node = graph
-          .parent_of_by_key(best_node.key())
-          .ok_or_else(|| make_internal_report!("Parent node is expected, but not found"))?;
-      } else if best_node.key() == current_best_node.key() {
-        // The best node is the current node. Break.
-        break;
-      } else {
-        // The best node is child
-        current_best_node = graph.get_node(best_node.key())?;
-      }
-    } else if current_best_node.is_leaf()
-      && !current_best_node.is_root()
-      && current_best_node.payload().tmp.private_mutations.nuc_muts.is_empty()
-    {
-      current_best_node = graph
-        .parent_of_by_key(best_node.key())
-        .ok_or_else(|| make_internal_report!("Parent node is expected, but not found"))?;
-    } else {
-      break;
-    }
-    // update the private mutations to match the new 'current_best_node'. This involves
-    // in step 1 subtracting the shared mutations from the private mutations struct
-    private_mutations = difference_of_muts(&private_mutations, &best_split_result.shared).wrap_err_with(|| {
+    // Update query mutations to adjust for the new position of the placed node
+    private_mutations = update_private_mutations(&private_mutations, &candidate_split).wrap_err_with(|| {
       format!(
-        "When calculating difference of mutations between query sequence and the branch leading to the next attachment point '{}'",
-        current_best_node.payload().name
+        "When updating private mutations against the current best node '{}'",
+        best_node.payload().name
       )
     })?;
-    // in step 2 we need to add the inverted remaining mutations on that branch.
-    // Not that this can be necessary even if there are no left-over nuc_subs.
-    // Amino acid mutations can be decoupled from the their nucleotide mutations or
-    // changes in the amino acid sequences due to mutations in the same codon still need handling
-    private_mutations = union_of_muts(&private_mutations, &best_split_result.left.invert()).wrap_err_with(|| {
+  }
+
+  Ok((best_node.key(), private_mutations))
+}
+
+/// Check how many mutations are shared with the branch leading to the current_best_node or any of its children
+fn find_shared_muts<'g>(
+  graph: &'g AuspiceGraph,
+  best_node: &'g Node<AuspiceGraphNodePayload>,
+  private_mutations: &BranchMutations,
+  masked_ranges: &[NucRefGlobalRange],
+  params: &TreeBuilderParams,
+) -> Result<(&'g Node<AuspiceGraphNodePayload>, SplitMutsResult, f64), Report> {
+  let (mut candidate_split, mut shared_muts_score) = if best_node.is_root() {
+    // Don't include node if node is root as we don't attach nodes above the root
+    let candidate_split = SplitMutsResult {
+      left: BranchMutations::default(),
+      right: private_mutations.clone(),
+      shared: BranchMutations::default(),
+    };
+    (candidate_split, 0.0)
+  } else {
+    let candidate_split = split_muts(&best_node.payload().tmp.private_mutations.invert(), private_mutations)
+      .wrap_err_with(|| {
+        format!(
+          "When splitting mutations between query sequence and the nearest node '{}'",
+          best_node.payload().name
+        )
+      })?;
+    let shared_muts_score = score_nuc_muts(&candidate_split.shared.nuc_muts, masked_ranges, params);
+    (candidate_split, shared_muts_score)
+  };
+
+  // Check all child nodes for shared mutations
+  let mut candidate_node = best_node;
+  for child in graph.iter_children_of(best_node) {
+    let child_split = split_muts(&child.payload().tmp.private_mutations, private_mutations).wrap_err_with(|| {
       format!(
-        "When calculating union of mutations between query sequence and the branch leading to the next attachment point '{}'",
-        best_node.payload().name
+        "When splitting mutations between query sequence and the child node '{}'",
+        child.payload().name
       )
     })?;
+    let child_shared_muts_score = score_nuc_muts(&child_split.shared.nuc_muts, masked_ranges, params);
+    if child_shared_muts_score > shared_muts_score {
+      shared_muts_score = child_shared_muts_score;
+      candidate_split = child_split;
+      candidate_node = child;
+    }
   }
-  Ok((current_best_node.key(), private_mutations))
+  Ok((candidate_node, candidate_split, shared_muts_score))
+}
+
+/// Find out if the candidate node is better than the current best (with caveats).
+/// Return a better node or `None` (if the current best node is to be preserved).
+fn find_better_node_maybe<'g>(
+  graph: &'g AuspiceGraph,
+  best_node: &'g Node<AuspiceGraphNodePayload>,
+  candidate_node: &'g Node<AuspiceGraphNodePayload>,
+  shared_muts_score: f64,
+  left_muts_score: f64,
+) -> Option<&'g Node<AuspiceGraphNodePayload>> {
+  if candidate_node == best_node {
+    // best node is the node itself. Move up the tree if all mutations between
+    // the candidate node and its parent are also in the private mutations.
+    // This covers the case where the candidate is a leaf with zero length branch
+    // as the  .left.nuc_muts is empty in that case
+    if left_muts_score == 0.0 {
+      return graph.parent_of(candidate_node);
+    }
+  } else if shared_muts_score > 0.0 {
+    // candidate node is child node, move to child node if there are shared mutations
+    // this should always be the case if the candidate node != best_node
+    return Some(candidate_node);
+  }
+  // no improvement possible. Return None to stay
+  None
+}
+
+/// Update private mutations to match the new best node
+fn update_private_mutations(
+  private_mutations: &BranchMutations,
+  best_split_result: &SplitMutsResult,
+) -> Result<BranchMutations, Report> {
+  // Step 1: subtract shared mutations from private mutations
+  let private_mutations = difference_of_muts(private_mutations, &best_split_result.shared).wrap_err(
+    "When calculating difference of mutations between query sequence and the branch leading to the next attachment point"
+  )?;
+
+  // Step 2: We need to add the inverted remaining mutations on that branch.
+  // Note that this can be necessary even if there are no left-over nuc_subs.
+  // Amino acid mutations can be decoupled from the their nucleotide mutations or
+  // changes in the amino acid sequences due to mutations in the same codon still need handling.
+  let private_mutations = union_of_muts(&private_mutations, &best_split_result.left.invert()).wrap_err(
+    "When calculating union of mutations between query sequence and the branch leading to the next attachment point.",
+  )?;
+
+  Ok(private_mutations)
 }
 
 pub fn attach_to_internal_node(