“…Other reports of increased frequency of twins with OA support this association [8,13,27]. There have been 173 reported cases of OA in twins [1,4,8,13,15,26,27,31,34], of which 80% have occurred in same-sex pairs [13,27]. There are reports of 15 sets of twins concordant for the OA anomaly [4,8,15,26,27,34].…”