Immunomycin (Ascomycin, FR-900520, FK520) is a macrocyclic lactone produced by S. hygroscopicus. It is well known as an antifungal and antibiotic agent. Immunomycin is an analog of FK506 and also contains a 23-membered ring. Both FK506 and Immunomycin bind to FKBP12 at the same binding site for hindering calcineurin. Thus, Immunomycin possesses immunosuppressive activity similar to that of FK506. It appears that Immunomycin inhibits the production of Th1 (interferon- and IL-2) and Th2 (IL-4 and IL-10) cytokines. Additionally, Immunomycin preferentially inhibits the activation of mast cells, an important cellular component of the atopic response. Immunomycin produces a more selective immunomodulatory effect in that it inhibits the elicitation phase of allergic contact dermatitis but does not impair the primary immune response when administered systemically. Immunomycin is a member of the polyketide group of compounds. The polyketide synthase (PKS) enzyme is used to produce polyketides. The preparation of polyketide compounds has low product yield with traditional chemical methodology using wild-type cells. Reeves et al. provide a novel method of preparing a polyketide using a host cell with a recombinant DNA vector. The recombinant DNA vector contains nucleic acids that can encode the Immunomycin PKS enzyme. Immunomycin synthesized using this technology yields PKS at higher levels than that achieved by PKS in nature.