Pepsin crystals (X90) Northrop, 1930) Northrop,J.H.(1930) “Crystallin pepsin,.l: lsolation and tests of purity"J.Gen.Physiol. 13:739-766.Pepsin crystals (X90) Northrop, 1930) Northrop, J. H. (1930) “Crystallin pepsin, 1: Isolation and tests of purity” J. Gen . Physiol. 13:739-766