Although multiple RPTKs are known to be expressed in human breast cancer cells, synthetic inhibitors of four RPTKs – epidermal growth factor receptor (EGFR), HER/ErbB, platelet-derived growth factor receptor (PDGFR) and insulin receptors (IR and IGF-1R) – were used to investigate whether they up-regulate the activity of -575 p27 (p27-5'UTR). Here, IGF1R is linked to breast cancer.