Single-stranded DNA-specific cytidine deaminase. Involved in somatic hypermutation (SHM), gene conversion, and class-switch recombination (CSR) in B-lymphocytes by deaminating C to U during transcription of Ig-variable (V) and Ig-switch (S) region DNA. Required for several crucial steps of B-cell terminal differentiation necessary for efficient antibody responses (PubMed:18722174, PubMed:21385873, PubMed:21518874, PubMed:27716525). May also play a role in the epigenetic regulation of gene expression by participating in DNA demethylation (PubMed:21496894). {ECO:0000269|PubMed:18722174, ECO:0000269|PubMed:21385873, ECO:0000269|PubMed:21496894, ECO:0000269|PubMed:21518874, ECO:0000269|PubMed:27716525}. This is the function of ENSG00000111732 (AICDA, activation induced cytidine deaminase).