STR typing from punched FFPE tissues: resolving suspected contamination and fetal–maternal mixture in two cases

The analysis of mixed or contaminated biological materials represents a persistent challenge in both forensic genetics and diagnostic pathology. This study presents two casework applications demonstrating the analytical value of a targeted FFPE punching strategy coupled with STR genotyping for resolving complex scenarios. In the first case, an unexpected focus of high-grade urothelial carcinoma within a prostatic surgical specimen raised concerns regarding cross-sample contamination. STR profiling of microdissected punches—respectively containing the carcinoma and benign prostatic tissue—yielded fully concordant single-source profiles (LR = 2.5 × 10²¹), thereby unequivocally establishing a common donor origin and excluding inadvertent tissue transfer. In the second case, involving paternity assessment of mechanically aborted embryonic material from an alleged sexual assault, morphologically guided punching enabled selective isolation of chorionic villi from a grossly heterogeneous specimen. Although low-level maternal DNA was detected, mixture interpretation allowed reconstruction of a complete embryonic genotype, resulting in a conclusive paternity (LR = 2.2 × 10¹⁰). Altogether, these observations underscore the analytical robustness, cost-efficiency, and operational simplicity of punched- FFPE-based STR typing as a first-line approach for contamination assessment and targeted genotype recovery.