The principal proteins are alpha-lactalbumin, lactoferrin (apo-lactoferrin), IgA, lysozyme, and serum albumin. In an acidic environment such as the stomach, alpha-lactalbumin unfolds into a different form and binds oleic acid to form a complex called HAMLET that kills tumor cells. This is thought to contribute to the protection of breastfed babies against cancer. Non-protein nitrogen-containing compounds, making up 25% of the milk's nitrogen, include urea, uric acid, creatine, creatinine, amino acids, and nucleotides. Breast milk has circadian variations; some of the nucleotides are more commonly produced during the night, others during the day. Mother's milk has been shown to supply endocannabinoids (the natural neurotransmitters that cannabis simulates) 2-Arachidonoyl glycerol, anandamide, oleoylethanolamide, palmitoylethanolamide, N-arachidonoyl glycine, eicosapentaenoyl ethanolamide, docosahexaenoyl ethanolamide, N-palmitoleoyl-ethanolamine, dihomo-γ-linolenoylethanolamine, N-stearoylethanolamine, prostaglandin F2alpha ethanolamides and prostaglandin F2 ethanolamides, Palmitic acid esters of hydroxy-stearic acids (PAHSAs). They may act as an appetite stimulant, but they also regulate appetite so infants don't eat too much. That may be why formula-fed babies have a higher caloric intake than breastfed babies. Breast milk is not sterile, but contains as many as 600 different species of various bacteria, including beneficial Bifidobacterium breve, B. adolescentis, B. longum, B. bifidum, and B. dentium, which contribute to colonization of the infant gut. As a result, it can be defined as a probiotic food, depending on how one defines "probiotic". Breast milk also contains a variety of somatic cells and stem cells and the proportion of each cell type differs from individual to individual. The somatic cells are mainly lactocytes and myoepithelial cells derived from the mother's mammary glands. The stem cells found in human breast milk have been shown to be able to differentiate into a variety of other cells involved in the production of bodily tissues and a small proportion of these cross over the nursing infant's intestinal tract into the bloodstream to reach certain organs and transform into fully functional cells. Because of its diverse population of cells and multifarious functions, researchers have argued that breast milk should be considered a living tissue. Breast milk contains a unique type of sugars, human milk oligosaccharides (HMOs), which are not present in infant formula. HMOs are not digested by the infant but help to make up the intestinal flora. They act as decoy receptors that block the attachment of disease causing pathogens, which may help to prevent infectious diseases. They also alter immune cell responses, which may benefit