This hypothesis is corroborated by the evidence that the lung, one of the sites mostly colonized by ccRCC metastases, shows a lower level of arginine, both at tissue and interstitial fluid level, compared to the kidneys and that ASS1 expression is heterogenous in primary tumors, with the detection of single-cell clones positive for ASS1 even when ASS1 is largely suppressed. This evidence concerns the gene ASS1 and nonpapillary renal cell carcinoma.