KRAS and neoplasm: To address the hypothesis that alteration in the ratio of the K-Ras proto-oncoproteins in favour of K-Ras 4B can affect tumour formation in the small intestine in the absence of K-ras activating mutations, K-rastmΔ4A/tmΔ4A mice, which express the K-ras 4B splice variant only [18], were crossed with ApcMin/+ (Min) mice.