APOBEC3B (Apolipoprotein B mRNA editing enzyme catalytic subunit 3B) is a member of the human APOBEC (Apolipoprotein B mRNA editing enzyme catalytic polypeptide-like) family, which includes enzymes that deaminate cytosine (C) to uracil (U) in RNA and single-stranded DNA. APOBEC3B is a DNA cytosine deaminase that can convert cytosine in DNA to uracil. This conversion, if not repaired, can lead to C to T transition mutations during DNA replication. Its role in tumorigenesis in cancer development is achieved by promoting mutation processes. These mutations provide genetic diversity for cancer, thereby promoting cancer invasion, metastasis, recurrence, and therapeutic resistance.