Dihydroorotate dehydrogenase inhibitors downregulate MDSC generation and maturation, improving immunotherapy efficacy in an in vivo TNBC model [294] (Fig. 4B). Brequinar, an inhibitor of dihydroorotate dehydrogenase, has been reported to enhance the effectiveness of immune checkpoint inhibition in refractory HER2+ breast cancer [295]. The gene discussed is ERBB2; the disease is breast cancer.