By measuring copy number variants in 816 clubfoot probands, we identified duplications of chromosome Xp22.33 involving the SHOX gene and surrounding regions in 1.1% of cases compared to 0.07% of controls. These duplications are likely causative in some cases, as four out of six SHOX duplications were not present in the unaffected parents but were confirmed in the proband, resulting in a de novo rate of 66% (4/6) for chromosome Xp22.33 duplications. This will have immediate and direct impact on patient care because these genetic abnormalities can easily be detected with clinically available chromosomal microarray studies. (By Brooke Sadler, https://jmg.bmj.com/content/early/2020/06/09/jmedgenet-2020-106842 )