To understand the relationship between mt-PCPH catalytic activity and the reduced ATP levels and enhanced chemo-resistance conferred by mt-PCPH, ATP levels were determined in extracts of HCT116 and HCT15 CRC cells, which express low endogenous levels of PCPH or mt-PCPH, after stable transfection with empty vector (pcDNA) or myc-taggedmt-PCPH(Fig. 1A). Here, MYC is linked to colorectal carcinoma.