The c-Cbl protooncogene can function as a negative regulator of receptor protein tyrosine kinases (RPTKs) by targeting activated receptors for polyubiquitination and downregulation. This function requires its tyrosine kinase binding (TKB) domain for targeting RPTKs and RING finger domain to recruit E2 ubiquitin-conjugating enzymes. It has therefore been proposed that oncogenic Cbl proteins act in a dominant-negative manner to block this c-Cbl activity. In testing this hypothesis, we found that although mutations spanning the RING finger abolish c-Cbl-directed polyubiquitination and downregulation of RPTKs, they do not induce transformation. In contrast, it is mutations within a highly conserved alpha-helical structure linking the SH2 and RING finger domains that render Cbl proteins oncogenic. Thus, Cbl transformation involves effects additional to polyubiquitination of RPTKs that are independent of the RING finger and its ability to recruit E2-conjugating enzymes.