McDonald KA, et al. - Utilizing The Cancer Genome Atlas, researchers investigated how tumor heterogeneity correlate to immune cells in a real-world cohort. Intratumoral heterogeneity was projected via calculating mutant allele tumor heterogeneity; CIBERSORT was used to estimate immune cell compositions. Outcomes revealed less immune cell infiltration, less activation of the immune response, and worse survival in correlation to high heterogeneity tumors in breast cancer. Results thereby support the notion that selection pressure of tumor-infiltrating immune cells shapes the tumor heterogeneity.