Molecular Characterization and Genetic Significance of a Rare Type-I Triallelic Pattern Involving a 15.3 Microvariant at the D19S433 Locus.

Short tandem repeat (STR) analysis is the main reference standard in forensic genetics. Despite its reliability, rare genetic anomalies like triallelic patterns and microvariants challenge profile interpretation. This study characterizes an exceptional genetic finding identified during routine casework at the Research Institute of Forensic Science in Sofia, Bulgaria. DNA was isolated from dental remains of an unidentified male corpse and amplified using the Investigator® ESSplex SE Plus kit targeting 15 autosomal STR loci. A well-balanced male genotype was obtained across 14 loci; however, a rare triallelic anomaly emerged at D19S433 (19q12). Three distinct peaks were identified: standard alleles 13 and 14, and an off-ladder allele at 261.09 bp, designated as microvariant 15.3. Quantitative peak height analysis showed that the combined intensity of alleles 13 and 14 (~1316 RFU) was approximately equal to that of microvariant 15.3 (1054 RFU). This balance confirms a Type I triallelic configuration, indicative of a localized somatic mutation or segmental duplication rather than trisomy or DNA mixture. Global databases (NIST STRBase and STRidER) and regional Bulgarian data confirmed that this specific 13/14/15.3 combination has not been previously documented. Documenting rare STR variations is critical to enriching genomic databases and ensuring maximum accuracy in forensic interpretations.