Mutant KRAS favors a microbe that signals back.

Tumor genetics and the microbiome both shape colorectal cancer, but their reciprocal interplay remains poorly defined. In this issue of Cell Host & Microbe, Jiao et al. show that mutant KRAS favors Bacteroides acidifaciens, which signals back through a SusF-ARHGEF2 axis to reinforce RAS-MAPK signaling.