RB1 and Williams syndrome: However, in the cell samples from the DGS/VCFS and the Williams syndrome patients, the frequency of SD cells for the imprinted SNRPN locus were significantly lower than the corresponding values for RB1 (P < 0.001 for the DGS/VCFS patients and P < 0.0002 for those with Williams syndrome; Figure 3c, d, e, f).