Feeding animals CD supplemented with 100 mM of stable isotope of L-proline (13C15N-proline) led to increased proline in ras1G12V;csk−/−,path tumours but not in ras1G12V,CG1139;csk−/− tumours (Fig. 5j), revealing that Path mediates tumour uptake of circulating proline. This evidence concerns the gene CSK and neoplasm.