Multistep microsatellite mutation leading to father–child mismatch of FGA locus in a case of non-exclusion parentage

•A non-exclusion paternity case revealed three-/four-step mutation at FGA locus.•The alleles in the father, the mother and the child are 18/25, 20/26, and 20/22.•The combined paternity index of 39 autosomal STRs is up to 1.3618×106.•The haplotype of 16 Y-STR loci in the child matched with that of the father.