Since CD8+ T cells are known to play an integral part of the anti-tumour response9 and were decreased in 65-week-old PBS treated Mdr2−/− mice (Fig. 1E), the significantly increased number of CD8+ T cells in aged Mdr2−/− mice treated with CoPP may indicate a less tumour susceptible microenvironment. Here, ABCB4 is linked to neoplasm.