As shown in Fig. 1a–d, although CDKN2A expression could be readily detected in two low-CIC cell lines (HEK293 and ZR75-1), it is undetectable in human breast cancer cell MCF7 and non-transformed mammary epithelial cell MCF10A, two cell lines that could form high frequency of CIC structures, suggesting a negative role of CDKN2A in CIC formation. This evidence concerns the gene CDKN2A and breast cancer.