RB1 and Williams syndrome: Similarly, the frequency of SD cells in the samples from the Williams syndrome patients ranged from 12% to 31% for the SNRPN locus and from 29% to 51% for the RB1 locus with a mean of 23.4 ± 6.5% and 36.2 ± 6.7%, respectively (Figure 3e and Figure 3f).