Paternity tests are carried out by the analysis of hypervariable short tandem repeat DNA loci. These microsatellite sequences mutate at a higher rate than that of bulk DNA. The occurrence of germline mutations at STR loci posses problems in interpretation of resulting genetic profiles. We recently analyzed 59-159 parent/child allele transfers at 13 microsatellite loci. We identified 12 mutations in 7 microsatellite loci. No mutations were occurred in other 6 loci. The highest mutation rate was observed with 5 mutations at D8S1179 locus at different alleles. The event was always single repeat related. The mutation rate was between 0 and 1.5 x 10(-2) per locus per gamete per generation. The mutation event is very crucial for forensic DNA testing and accumulation of STR mutation data is extremely important for genetic profile interpretation.