the addition of protamine obtained from the "milt" or semen of river trout. The insulin would be added to the protamine, but the solution would have to be brought to pH 7 for injection. University of Toronto, Canada later licensed protamine zinc insulin (PZI), to several manufacturers. This mixture only needs to be shaken before injection. The effects of PZI lasted for 24–36 h. In 1946, Nordisk was able to form crystals of protamine and insulin and marketed it in 1950, as neutral protamine Hagedorn (NPH) insulin. NPH insulin has the advantage that it can be mixed with an insulin that has a faster onset to complement its longer lasting action. Eventually all animal insulins made by Novo Nordisk were replaced by synthetic, recombinant 'human' insulin. Synthetic 'human' insulin is also complexed with protamine to form NPH. Timeline The timeline is as follows: 1926 Nordisk receives Danish charter to produce insulin 1936 Hagedorn discovers that adding protamine to insulin prolongs the effect of insulin 1936 Canadians D.M. Scott and A.M. Fisher formulate zinc insulin mixture and license to Novo 1946 Nordisk crystallizes a protamine and insulin mixture 1950 Nordisk markets NPH insulin 1953 Nordisk markets "Lente" zinc insulin mixtures. Society and culture Names NPH stands for neutral protamine Hagedorn, and the words refer to neutral pH (pH = 7), protamine (a protein), and Hans Christian Hagedorn (an insulin researcher).Brand names include Humulin N, Novolin N, Novolin NPH, Gensulin N, SciLin N, Insulatard, and NPH Iletin II. See also Insulin analogue References Insulin therapies Human proteins Recombinant proteins Peptide hormones Eli Lilly and Company brands World Health Organization essential medicines Wikipedia medicine articles ready to translate Medical mnemonics