Somatic hypermutation (SHM) of immunoglobulin variable (V) regions modulates antibody-antigen affinity is initiated by activation-induced cytidine deaminase (AID) on single-stranded DNA (ssDNA).

Figure 1: Mechanism of nucleotide excision repair and base excision repair. The protein factors that are required for TCR and general genome repair (GGR) of DNA damage fall into two broad groups.