Phosphorylation of Rb in BLIN-2 cells cultured in the presence or absence of BM stromal cells. BLIN-1 or BLIN-2 cells were cultured in X-VIVO 10 serum-free medium for 18 hours in the absence (−) or presence (+) of BM stromal cells. The cells were then harvested and lysed in 0.5% NP-40, and approximately 50 μg of protein per lane was electrophoresed on a 10% SDS-PAGE gel. The separated proteins were then transferred to nitrocellulose and Western blotted with rabbit antihuman Rb. The blot was restained with mouse antihuman β tubulin to control for equal protein loading. The numbers under each lane represent the percentage of Rb detected as the hyper-phosphorylated isoform (ppRb ÷ pRb + ppRb) by scanning densitometry.