PDCD1 and breast cancer: Using mouse models of mammary cancer that elicit robust MDSC responses, we demonstrated that the DHODH inhibitor brequinar (a) suppressed MDSC production from early-stage myeloid progenitors, which was accompanied by enhanced myeloid maturation; (b) augmented the antitumor and antimetastatic activities of programmed cell death 1–based (PD-1–based) ICI therapy in ICI-resistant mammary cancer models; and (c) acted in concert with PD-1 blockade through modulation of MDSC and CD8+ T cell responses.