To further evaluate the basis for the variable tumour behaviour exhibited by β1 integrin-deficient lesions, IHC analyses showed that emerging tumours had dramatic reduced levels of key proliferation markers Ki67, cyclin D1 and phospho-Rb (Fig. 4c–e), which is consistent with the early β1 integrin-deficient lesions (Fig. 2c, d). This evidence concerns the gene CCND1 and neoplasm.