MEN1 and neoplasm: Our studies of genetic background on endocrine tumour formation in adult Men1+/- mice has revealed that the tumours observed and their age of onset, in the 129S6/SvEv and C57BL/6 congenic Men1+/- mouse strains, were comparable to that in the same model on a mixed 129S6/SvEv and C57BL/6 background, with less than five consecutive backcrosses since the generation of the original chimera (19) to that described in other reported Men1 conventional mouse knockout models on a pure 129S6/SvEv background (29) or mixed backgrounds (Supplementary Table 4) (19, 30, 31).