To further broaden our investigation into the mechanism ofRP1 functioning in breast cancer cells, werestored p27kip1 in MCF-7 and SKBR3 cells with overexpressed RP1. BrdU assays showed that p27kip1 significantlyinhibited the proliferation of MCF-7 and SKBR3 cells with overexpressed RP1 (Fig. 4a).Conversely, we repressed p27kip1 expression in MDA-MB-231 and BT549 cells withRP1 knockdown, and showed that theproliferation of these cells was remarkably enhanced compared with each controlgroup (Fig. 4b). Here, RP1 is linked to breast cancer.