The purpose of this study is to determine the prevalence of germline APOBEC3B deletion and its association with breast cancer risk in a multi-ethnic Asian cohort of Chinese, Malay and Indian women in Malaysia, to compare the gene expression profiles of breast cancers arising from Asian APOBEC3B deletion carriers and non-carriers, and lastly, to characterise the presentation of tumour-infiltrating immune cells in breast cancers. Here, APOBEC3B is linked to breast cancer.