Abstract: Objective To study the sequences of off-ladder alleles of PowerPlexTM16 kit in Chinese Han population and their nomenclature. Methods 10071 Samples from unrelated individuals in Chinese Han population were screened by using PowerPlexTM16 kit and ABI 377 or 3100. The samples showing off-ladder alleles were re-screened with PAGE and the off-ladder alleles were obtained and sequenced. Results 32 off-ladder alleles were found in 11 STR loci, whose frequencies ranged from 0.05‰ to 4.02‰. These alleles were classified as four types: (1)having complete repeat but its length is out of the ladder; (2)having incomplete repeat; (3)having deletion or insertion of one or two base(s) in flanking sequence; (4) having deletion of some segment. Conclusion Off-ladder alleles have various types. It demonstrates varying repteating number of the core unit, and variation in the flanking sequence or core sequence as well.The nomenclature of International Society for Forensic Haemogenetics cannot define these alleles effectively.

Key words: short tandem repeat, off-ladder allele, PowerPlexTM16 kit