The invention provides a nucleic acid molecule comprising: (a) a nucleotide sequence as shown in SEQ ID No. 35; or (b) a nucleotide sequence which is the complement of SEQ ID No. 35; or (c) a nucleotide sequence which is degenerate with SEQ ID No. 35; or (d) a nucleotide sequence hybridising under conditions of high stringency to SEQ ID No. 35, to the complement of SEQ ID No. 35, or to a hybridisation probe derived from SEQ ID No. 35 or the complement thereof; or (e) a nucleotide sequence having at least 80% sequence identity with SEQ ID No. 35; or (f) a nucleotide sequence having at least 65% sequence identity with SEQ ID No. 35 wherein said sequence preferably encodes or is complementary to a sequence encoding a nystatin PKS enzyme or a part thereof. Also provided are part of such molecules and polypeptides (and parts thereof) encoded by such a nucleic add molecule, and the use of such molecules and polypeptides in facilitating nystatin biosynthesis and in the synthesis of nystatin derivatives and novel polyketide as macrolide structures.