Prader-Willi syndrome (PWS) is the most common known cause of life-threatening obesity due to errors in genomic imprinting from chromosome 15 defects. We summarized the frequency and characterized the PWS molecular classes using advanced genomic technology in 510 individuals  as the largest cohort to date (paternal 15q11-q13 deletion in 60% of cases, maternal disomy 15 (UPD15) in 36% and imprinting defects in 4%) and studied maternal age effects. The PWS child with UPD15 receives both chromosome 15s from the mother and may have a second genetic condition if the mother carries a recessive mutation in an isodisomic region on chromosome 15. Significantly older mothers were found in the UPD15 group and age may contribute to both 15s from the mother. (By Professor Merlin G. Butler, http://jmg.bmj.com/content/early/2018/05/05/jmedgenet-2018-105301 )