= Solution
The repetitive tract can cause <DNA polymerase slippage> during <PCR amplification> or the sequencing extension itself, generating stutter products. A heterozygous length variant creates overlapping <Sanger sequencing> traces downstream of the <indel> because the two templates lose register; this does not by itself indicate random sequencing failure. Tumor heterogeneity or contamination by normal tissue can add further mixed templates. \b[Avoid creating or obscuring the mutation during amplification and avoid interpreting a mixed trace as one clean allele.] Use a suitable high-fidelity <DNA polymerase>, independent amplifications and bidirectional reads, and resolve individual alleles by molecular cloning or an appropriate single-molecule/amplicon sequencing method if necessary. Check for non-specific products and allelic dropout caused by variation in a primer-binding site.
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