CSA (Cockayne syndrome type A protein) is another substrate receptor for CUL4–DDB1, and CRL4CSA is recruited to TC-NER sites where it is believed to ubiquitylate the SWI/SNF ATPase CSB (Cockayne syndrome type B protein) and is important for repair and ensuing transcriptional re-start [266,302]. Here, DDB1 is linked to Down syndrome.