Triple-negative breast cancer has the worst prognosis of the breast cancer subtypes. Aggressive forms of this cancer show elevated signaling through the transcription factor MYC, but blocking MYC activity remains challenging because of its role in normal cell function. Horiuchi et al. screened the protein kinases expressed by triple-negative breast tumors in hopes of finding a way to thwart MYC indirectly. They identified PIM1, a nonessential protein kinase that was highly active in MYC-positive tumors. Genetic depletion of PIM1 promoted cancer cell death, and preclinical drugs targeting PIM1 impaired the growth of MYC-positive patient tumors in mice. These findings pave the way for the development of PIM1 inhibitors in early-phase clinical trials.