Two other transcription factors, used as negative control, Myb and
LEF-1, showed no effect of overexpression on ABCB10 promoter activity
(Fig. 3A).Moreover,we did not observe any effect of overexpressing the
pocket proteins RB1, RBL1, and RBL2, even when we cotransfect them
together with E2F2/4 (Supplementary Fig. 1). Since these proteins are
negative regulators of E2Fs by masking their transactivation domain
[23] resulting in inhibitory action of Rb-E2F complexes, the lack of
modulatory function of Rb proteins in our model supports the notion
that E2F TFs function as free activators of transcription [22] of the
ABCB10 gene