Using a combination of site-directed mutagenesis, immunoprecipitation methods,in vitroenzyme activity assays andin situenzyme activity determinations in live cells, this report also demonstrates that the mt-PCPH oncoprotein lacks detectable NTPDase activity, indicating that direct ATP cleavage by mt-PCPH did not cause the ATP depletion observed in mt-PCPH-expressing CRC cells. This evidence concerns the gene ENTPD5 and colorectal carcinoma.