Abstract: Objective To explore the feasibility of improving the sensitivity of DNA detection by increasing the PCR cycle index and decreasing the volume of amplifying system. Methods The DNA of semen were collected from 10 healthy irrelevant volunteers, and were quantified to 50, 40, 30, 25, 20, 15, 10 pg/μL, separately. All samples were then amplified in 10, 5, 3 μL volume and at 28, 30, 32, 34, 36 cycles, respectively. 3130 genetic analyzer was used to detect 15 autosomal STR loci. Results Under the situation of 28 cycles and 3 μL volume, samples which achieved >40 pg/μL could be correctly typed. Under the situation of 10, 5, 3 μL volume, samples which achieved >20 pg/μL could be correctly typed at 34 cycles. When increasing the index to 36 cycles, they could not be correctly typed because of the non-specific band. Conclusion DNA detecting sensitivity can be improved to a certain extent by increasing the cycle index and decreasing the volume of amplifying system.

Key words: forensic genetic, STR, semen, cycle index