Abnormal heart rhythms (AHRs) are associated with an increased risk of blood clots, heart failure, stroke and dementia, and risk of being affected appears to be increased in individuals with a genetic deletion on the short arm of the X chromosome (Xp22.31). We analysed data from UK Biobank and an online survey to characterise the features of deletion-associated AHRs. We show that such AHRs may be precipitated by stress, and are often comorbid with gut problems, asthma and anaemia. Loss of the STS (steroid sulfatase) gene specifically may underlie the increase in AHR risk. These findings: should improve genetic counselling for Xp22.31 deletion carriers, highlight particular carrier subgroups for cardiac screening, and suggest that research in STS-deficient individuals or cells will clarify biological mechanisms underlying AHRs. (By Dr William Davies, https://jmg.bmj.com/content/early/2022/11/15/jmg-2022-108862 )