Abstract: Biologically active aprotinin (APR) molecules, naturally occurring, relatively short, single chain polypeptides, are prepared by growing methylotrophic yeast transformants containing in their genome at least one copy of a DNA sequence operably encoding APR, in operational association with a DNA sequence encoding the S. cerevisiae alpha mating factor pre-pro sequence (including a processing sequence selected from lys-arg-(glu-ala).sub.x, wherein x is an integer falling in the range of 0-3), both under the regulation of a promoter region of a gene of a methylotrophic yeast, under conditions allowing expression of said DNA sequences, and secretion of APR molecules into the culture medium.Also disclosed are novel DNA fragments and novel recombinant yeast strains which are useful in the practice of the present invention.