Defective TGF-β growth arrest in Rb1ΔL/+ fibroblasts. (A) The level of pRB expression was measured in Rb1+b1+/+, Rb1ΔL/+/+ and Rb1ΔL/ΔLfibroblasts by western blotting. The upper blot shows pRB expression in these extracts and the lower blot shows Actin as a loading control. (B) All cells were treated with TGF-β for 24 hours, pulse labelled with BrdU for 1.5 hours and analyzed for incorporation by flow cytometry to determine the percentage of positive cells. The mean percentage of BrdU positive cells was determined from three independently treated cultures of cells. Means were compared between genotypes using a t-test and were found to be statistically significant (* P < 0.05).