Given that re-expression of ASS1 levels with corresponding increase of argininosuccinate was a crucial event for metastatic ccRCC cells to invade in vitro and metastasise in vivo in a mouse model42, these results may indicate that in some primary tumours, cells may re-express ASS1, becoming primed to metastasise. This evidence concerns the gene ASS1 and nonpapillary renal cell carcinoma.