Kefir originate from Caucasus Mountains (Eastern Europe) is a probiotic beverage (yogurt like) made with milk fermented with Kefir grains. http://en.wikipedia.org/wiki/Kefir
Kefir grains contains live active bacteria and yeast mixture clumped together, living in symbiosis resulting in the fermentation of the milk.
Kefir contains 3x the amount of probiotic cultures than yogurt, and the beneficial bacteria actually colonize the intestinal tract, something yogurt cannot match.
There are many publications that can be found at National Center for Biotechnology Information (NCBI, USA) that Kefir contain constituents that specifically inhibit growth of cancer cells. http://www.ncbi.nlm.nih.gov/pubmed/17887934