ERCC6 and Cockayne syndrome: In the same vein, dermal fibroblasts from a Cockayne syndrome patient (carrying a mutation in the ERCC6 gene resulting in the loss of a functional CSB protein) were dramatically more vulnerable to UVB irradiation, in addition to the formation of a lower number of colonies after their repetitive exposure to their highest non‐cytotoxic UVB radiation dose, compared to HDFs from normal donors (Figure 5e and Figure S4).