TRRAP links Myc with histone acetylases and appears to be an important mediator of its oncogenic function. Here we show that interaction with TRRAP is required for cellular transformation not only by Myc, but also by the adenovirus E1A protein. Substitution of the 262 N-terminal residues of Myc with a small domain of E1A (residues 12–54) restores Myc transforming function. E1A (12–54) contains a TRRAP-interaction domain, that recruits TRRAP to either E1A–Myc chimeras, or the native 12S E1A protein. Overexpression of a competing TRRAP fragment in vivo blocks interaction of cellular TRRAP with either E1A–Myc or E1A, and suppresses cellular transformation by both oncoproteins. Moreover, E1A (Δ26–35) that fails to bind TRRAP but is capable of binding the Retinoblastoma (Rb)-family and p300/CBP proteins is defective in cellular immortalization, transformation and cell cycle deregulation. Thus in addition to disrupting Rb and p300/CBP functions, E1A must recruit TRRAP to transform cells.