Parental homozygous LoxP Rbf/f MEFs (Marino et al., 2000) were treated with TAT-Cre protein (Wadia et al., 2004) to delete Rb (Rb−/−), followed by infection with carefully engineered and titered wild type (WT) Rb-HA or non-phosphorylatable ΔCdk Rb-HA retroviruses (Figure 5A). Here, RB1 is linked to infection.