Overexpression of Bcl-xL, loss of p19 ARF, and loss of p53 all accelerate Myc oncogenesis. All three lesions are implicated in suppressing Myc-induced apoptosis, suggesting that this is a common mechanism by which they synergize with Myc. However, using an acutely switchable model of Myc-induced tumorigenesis, we demonstrate that each lesion cooperates with Myc in vivo by a distinct mechanism. While Bcl-xL blocks Myc-induced apoptosis, inactivation of p19 ARF enhances it. However, this increase in apoptosis is matched by increased Myc-induced proliferation. p53 inactivation shares features of both lesions, partially suppressing apoptosis while augmenting proliferation. Bcl-xL and p19 ARF loss together synergize to further accelerate Myc oncogenesis. Thus, differing lesions cooperate oncogenically with Myc by discrete mechanisms that can themselves synergize with each other.