TDO2 and neoplasm: Du R et al. [250] used scRNA-seq to identify a critical cellular component of drug resistance out of trastuzumab, PDPN+CAF, that CAF induces tumor resistance by secreting indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase 2 (TDO2), which inhibit the killing ability of NK cells against tumor cells.