HAAO and renal cell carcinoma: Examination of a parallel existing LC-MS/MS proteomic data set generated by our group as part of an ongoing proteogenomic study in RCC not only confirmed loss of QPRT in RCC but also showed loss of five other enzymes in the kynurenine pathway, namely HAAO, KMO, kynurenine formamidase and KYAT1 and 2.