The adenovirus early gene product E1A is a potent stimulator of cellular proliferation, which when overexpressed can overcome the growth-inhibitory effects of TGF-beta. The E1A region encodes a series of related proteins (35-46kDa) with multifunctional capabilities and form a specific complex with the retinoblastoma tumor suppressor gene product (Rb protein). The E1A and E1B regions together comprise the transforming region of adenovirus. While expression of E1A alone is sufficient to immortalize primary cells, complete transformation requires the additional expression of the E1B region.