Since 1995 the Forensic Science Service (FSS) has carried out DNA profiling of reference samples for the UK National DNA Database (1) and in forensic casework using two multiplex STR profiling systems (2,3). During this period, profiles with anomalous banding patterns, although comparatively rare, have been encountered regularly. The FSS has collected instances of triallelic patterns and aberrant diallelic patterns. A systematic examination of these patterns has provided insight into their underlying genetic cause. The triallelic patterns could be classified into two types based on the relative intensities of their component alleles. In the Type 1 pattern the alleles were of uneven intensity, whereas in the Type 2 pattern, all three alleles were of even intensity. Evidence is presented that the more frequent Type 1 pattern is the result of somatic mutation at a heterozygous locus, and the Type 2 pattern is the result of a localized chromosomal rearrangement at a heterozygous locus. Directly from the Type 1 pattern, it was possible to deduce the size difference between the progenitor and mutated allele. All mutational changes were found to be multiples of four nucleotides, suggesting the loss or addition of one or more tetrameric repeat units. Aberrant diallelic patterns were identified by analysts due to an unexpectedly large difference in intensity between alleles at a heterozygous locus. While some of these diallelic patterns are likely caused by the same genetic phenomena described above occurring at a homozygous locus, others are demonstrated to be caused by a mutation in the primer binding sequence, leading to a reduction in amplification efficiency of one allele. It is concluded that based on a visual inspection of a profile, it is possible to infer a likely genetic basis directly from the triallelic pattern. By contrast, the aberrant diallelic patterns can be due to any one of a number of possible genetic effects.