Protein profiles of milk from bovine, caprine, human and equine species after digestion in two steps. Step 1: HGJ at pH 2.5 for 30 min/37 °C. Step 2: HDJ at pH 8.0/37 °C for 30 min. 1: Low molecular standard, 2: Undigested milk, 3: HGJ for 30 min, and 4–6: HGJ for 30 min followed by HDJ for 5, 15 and 30 min, respectively. Band within circle is uniquely produced in human milk and seems to be a lower molecular weight fragment of the casein band. α-la = α-lactalbumin, β-lg = β-lactoglobulin, CNs = caseins, HDJ = human duodenal juice, HGJ = human gastric juice, IgHc = immunoglobulin heavy chain, LF = lactoferrin, LZ = lysozyme and SA = serum albumin.