RB1 and breast carcinoma: Unlike the Rb Ser567 phosphorylation-mediated and p-38 signaling pathway-activated induction of ubiquitin-dependent degradation of Rb in melanoma cells11, we observed that Rb was phosphorylated on Ser807/811 and subjected to ubiquitin-dependent degradation through a Smad-independent PI3K/AKT signaling pathway in BMP-2-activated breast cancer cells.