Chymosin (Rennin EC 3.4.23.4), an aspartyl proteinase, is the major proteolytic enzyme in the fourthstomach of the unweaned calf, and it is formed by proteolytic activation of its zymogene, prochymosin.Following the cloning of synthesized cDNAs on mRNA pools extracted from the mucosa of the calf fourthstomach, we have identified an alternatively spliced form of preprochymosin ...