A study of obstetrical records of COS probands indicated no increase in adverse pre or perinatal events for these cases compared with obstetrical records of their siblings (Nicolson et al submitted). In contrast, several findings point to increased risk for these probands. To date, a total of 5 (10.4%) COS subjects were found to have previously unknown cytogenetic abnormalities (Microdeletion of 22q11 (3 cases), (Usiskin et al, submitted), Mosaic 45X0 (one case) (Kumra et al, 1998) and balanced 1:7 translocation (Gordon et al 1994).