Plasma thymidine was significantly lower in SCID mice with AsPC-1 (p < 0.0001) or MiaPaCa-2 tumours (p = 0.0016) than in non-tumour bearing mice, but higher in mice with KPC tumours than in the PC mice, which lack the KRAS mutation and do not develop spontaneous tumours (p = 0.006). This evidence concerns the gene KRAS and neoplasm.