We first analyzed the kynurenine production in a panel of chemosensitive and multidrug resistant cancer cells, showing a different pattern of ABC transporters (S1 Fig): HT29, A549, K562, Met5A were human chemosensitive cells; HT29/dx, A549/dx and K562/dx were models of acquired MDR; HMM and JC were human and murine constitutively chemoresistant cells, respectively. Here, ABCG2 is linked to cancer.