The early region (E1A) of the adenovirus genome plays a central role in cellular transformation and regulation of gene expression. 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. Several conserved regions of E1A are similar to portions of other viral oncoproteins like the HPV- 16 and HPV-18 E7 and SV40 large T antigen.