Butler MG, et al. - Using high-resolution microarrays, comprehensive chromosome 15 genotyping and methylation-specific multiplex ligation probe amplification, experts intended to elucidate and characterize Prader-Willi syndrome (PWS) molecular classes and maternal age effects. Data further characterised UPD15 findings into subclasses based on the presence (size, location) or absence of loss of heterozygosity (LOH). Furthermore, the UPD15 group presented with significantly older mothers compared with the deletion subtype. Results depicted that LOH patterns in UPD15 could exert an impact on the risk of having a second genetic condition if the mother carried a recessive mutant allele in the isodisomic region on chromosome 15. It was also deduced that the risk of UPD15 could increase with maternal age.