diff --git a/src/SequenceVariation.jl b/src/SequenceVariation.jl
index 8ad2972..3bb4f73 100644
--- a/src/SequenceVariation.jl
+++ b/src/SequenceVariation.jl
@@ -480,6 +480,14 @@ function _refbases(d::Deletion, reference::S, pos::UInt) where S <: BioSequence
     end
 end
 
+function _altbases(d::Deletion, reference::S, pos::UInt) where S <: BioSequence
+    if pos == 1
+        return S([reference[pos+1]])
+    else
+        return S([reference[pos-1]])
+    end
+end
+
 export Insertion,
     Deletion,
     Substitution,