Spinal and bulbar muscular atrophy (Kennedy’s disease) is an X-linked motor neuron disorder caused by a CAG repeat expansion in the androgen receptor gene. Traditionally, alleles with 38 or more CAG repeats are considered fully penetrant in male carriers. By comparing, through systematic review and meta-analysis, the distribution of CAG repeat sizes between 2,576 patient alleles and 112,248 control alleles, we revealed an unexpectedly high frequency of expanded, disease-associated alleles in the general population. We suggest that the currently held view of an abrupt rise in penetrance at around 38 repeats should be revised for a more gradual rise. (By Dr Georgios Koutsis, https://jmg.bmj.com/content/early/2020/06/22/jmedgenet-2020-106963 )