IDO1 and invasive breast carcinoma: Furthermore, assessing the levels of IDO1 protein expression in BrCa tissues from cohort 2 showed that IDO1 was highly expressed in TN and HER2-enriched BrCa subtypes (F(2, 27) = 5.52; Fig. 2c) but was absent (or minimally detected) in luminal BrCa, results which were consistent with the serum KP metabolite profiles in cohort 1.