APOBEC1 and breast cancer: In addition, with respect to several APOBEC family members (including APOBEC3A, APOBEC3H, APOBEC1, APOBEC2, APOBEC4, and AICDA) showing no or extremely low expression levels in both ER+ and ER− breast cancers, we did not find any enrichment of repressive chromatin modifications (H3K9me3 and H3K27me3) in these genes (Fig. 1c and Additional file 2: Figure S2), suggesting some additional molecular mechanisms probably participate in the regulation of these genes.