Eighty per cent (4 of 5 males and 4 of 5 females) of WT mice developed gallstones when fed a lithogenic diet (LD) for 8 weeks whereas the penetrance in OPN−/− mice was 10% (1 of 5 males and 0 of 5 females) (Table 1). The gene discussed is SPP1; the disease is gallstones.