PHIP1 is a novel downstream transcriptional co-regulator of insulin-like growth factor-I receptor (IGF-IR), a tyrosine kinase receptor that is often elevated and autophosphorylated in breast cancer. In this study, I show that PHIP1 is upregulated in MCF10A cells stably overexpressing IGF-IR signaling components and that knock-down of PHIP1 significantly inhibits breast cancer cell proliferation by inducing transcriptional upregulation of p21 and downregulation of cyclin D2. I also show that stable overexpression of PHIP1 in MCF10A cells can lead to its proteasomal degradation. Together, our data indicate that PHIP1 is implicated in breast cancer cell growth and suggest a number of avenues that await exciting discovery.