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Short tandem repeats (STRs) loci are the genetic markers used for forensic human identity test. With multiplex polymerase chain reaction (PCR) assays, STRs are examined and measured PCR product length relative to sequenced allelic ladders. In the repeat region and the flanking region of the commonly-used STR may have DNA sequence variation. A mismatch due to sequence variation in the DNA template may cause allele drop-out (i.e., a "null" or "silent"allele) when it falls within PCR primer binding sites. The STR markers were co-amplified in a single reaction by using commercial PowerPlex^® 16 system and AmpFlSTR® Identifiler^® PCR amplification kits. Separation of the PCR products and fluorescence detection were performed by ABI PRISM^® 3100 Genetic Analyzer with capillary electrophoresis. The GeneMapperTM ID software were used for size calling and analysis of STR profiles. Here, this study described a forensic human identity test in which allelic drop-out occurred in the STR system D18S51. During the course of human identity test, two samples with a homozygous (16, 16 and 21, 21) null genotype at D18S51 locus were discovered using the PowerPlex^® 16 system. The loss of alleles was confirmed when the samples were amplified using AmpFlSTR^®Identifiler^® PCR amplification kit and resulted in a heterozygous (16, 20 and 20, 21) genotype at this locus each other. This discrepancy results suggest that appropriate measures should be taken for database comparisons and that null allele should be further investigated by sequence analysis and be reported to the forensic community.