Regulation of cell cycle markers in Rat-1 stable cells. Rat-1 cell lines stably expressing LMP1, Id1, or Id3 were analyzed by western blotting. Total cell lysates from control (pBabe), LMP1, myc-tagged Id1 (M3-Id1), or myc-tagged Id3 (M3-Id3) were analyzed for levels of p27, Cdk2 and Rb protein. Actin was used as a loading control and expression of constructs was confirmed by blotting with LMP1-specific and myc-tag antibodies. Fifty micrograms of protein were loaded per lane.