Mammary tumors in ruminants The frequency of mammary neoplasia in different species varies tremendously. Among women, breast cancer features as one of the most frequent types of cancer globally. Breast cancer is the cause of almost half a million deaths worldwide each year. Of these deaths, more than 58 % are occurring in developing countries. Mammary tumors are rare in cows, mares, goats, ewes, and sows as evident by few cases reported in literature. In dairy cattle and milking goats the udders are inspected daily by palpation when milking and the tumors would be detected soon. The morphological features of bovine mammary gland are more similar to those in human than in rodents, the latter being commonly used for cancerogenic studies. There are several factors that might affect the high rate of tumor development in women and attenuate the development of tumors in ruminants. Besides diet, life style, reproductive history, stress load, the general physiology and metabolism might have an influence. Translational research using different mammary gland cell populations from several species may lead to comparative studies and may help reduce the risk and contribute to discovery of new therapeutic targets to treat breast cancer. In present work the zero occurrence of mammary gland tumors in ruminants will be reviewed focusing on literature records and mechanisms underlying the resistance. Tumorji mlečne žleze pri prežvekovalcih Pogostost neoplazij v mlečnih žlezah je različna pri posameznih vrstah. Rak dojke je najbolj pogost tip raka pri ženskah. Vsako leto zaradi raka dojke umre približno pol milijona žensk. 58 % smrti je v deželah v razvoju. Tumorji mlečne žleze so redki pri kravah, kobilah, kozah, ovcah in svinjah, na to nakazuje malo opisov v literaturi. Pri kravah mlekaricah in kozah vime otipajo vsak dan med molzenjem in bi tako tumorje hitro odkrili. Morfološko je kravja mlečna žleza bolj podobna človeški kot glodavski, čeprav slednje pogosto uporabljajo v kancerogenih študijah. Več faktorjev vpliva na razvoj tumorjev pri ženskah oziroma preprečuje nastanek tumorjev pri prežvekovalcih. Poleg prehrane, načina življenja, reproduktivne zgodovine, stesa, imata najbrž vpliv tudi splošna fiziologija in metabolizem. Primerjalne študije večih celičnih populacij iz mlečnih žlez različnih vrst, lahko vodijo k odkritju novih terapevtskih sredstev in zmanjšanju tveganja za razvoj raka. V našem delu bomo pregledali literaturo in se osredotočili na mehanizme, ki so odgovorni za nizek pojav raka v prežvekovalskih mlečnih žlezah.