APOBEC3B and carcinoma: Although most cancers that display enrichment for APOBEC mutational signatures are carcinomas, i.e. tumours that arise from epithelial cells, little is known about the expression patterns and physiological regulation of APOBEC3A and APOBEC3B in healthy epithelium, or regarding the proportion of malignant cells that express APOBEC3A, APOBEC3B or both genes in tumour biopsies.