Mutagenesis by deamination is thought to play an important role in human colorectal cancer (CRC); CRC has a high frequency of mutations at NpCpG sequences (so-called signature 1 mutations), and half of TP53 inactivating mutations, which are critical alterations for CRC progression [14], are G:C to A:T transitions within a CpG site [5, 15–17]. The gene discussed is TP53; the disease is colorectal carcinoma.