Xanthine oxidoreductase (XOR) is the rate-limiting enzyme in purine catabolism occurring in most cell types. However, this housekeeping gene is expressed at very high levels in a number of mammalian tissues including the lactating mammary epithelium, suggesting additional roles for XOR in these tissues. Mice with targeted disruption of XOR were generated to assess these potential additional roles. XOR−/− mice are runted and do not live beyond 6 wk of age. Strikingly, however, XOR+/− females, although of healthy appearance and normal fertility, are unable to maintain lactation and their pups die of starvation 2 wk postpartum. Histological and whole-mount analyses showed that in XOR+/− females the mammary epithelium collapses, resulting in premature involution of the mammary gland. Electron microscopy showed that XOR is specifically required for enveloping milk fat droplets with the apical plasma membrane prior to secretion from the lactating mammary gland. We present evidence that XOR may have primarily a structural role, as a membrane-associated protein, in milk fat droplet secretion and thus XOR provides another example of "gene sharing". About 5% of women experience primary lactation insufficiency. The above observations suggest that human females suffering from xanthinuria, a deficiency in XOR, are potential candidates for lactation problems.