As next generation sequencers are widely used, somatic mosaicism is thought to be more common than previously expected. We report a mosaic patient with nevoid basal cell carcinoma syndrome, a genetic condition featured by minor anomalies and high prevalence of cancers such as basal cell carcinoma and medulloblastoma. The patient had a germline PTCH1 mutation, c.272delG, and a low prevalence somatic mutation, c.274delT. Since two mutations are located in close proximity, reversion error is likely to be involved in this event rather than a spontaneous mutation. This study is also unique in that mosaicism was initially identified by analysis of iPS cell clones derived from a single cell. (By Prof. Toshiyuki Miyashita, http://jmg.bmj.com/content/early/2017/03/31/jmedgenet-2016-104490 )