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The cAMP binding protein CABP1 isolated from Dictyostelium discoideum consists of two subunits, CABP1A and CABP1B, which are produced from a single gene by an unusual splicing mechanism. To characterize this mechanism, an actin-CABP1 fusion gene was constructed and introduced into D. discoideum cells by DNA-mediated transformation. Analysis of these cells demonstrated that the transcripts derived from the fusion gene were properly processed to generate both CABP1A and CABP1B. However, when the 5$ sp prime$ splice site in the gene was mutated to conform to the D. discoideum consensus, only CABP1B was produced. These results suggest that the splicing event is regulated by the 5$ sp prime$ splice site. By screening a cDNA library under conditions of reduced stringency with a sequence encoding CABP1, clones which code for two closely related molecules were isolated. Hybrid selection experiments indicated that these cDNAs encode polypeptides with molecular weights of 34,000 (p34) and 31,000 (p31), both of which are recognized by anti-CABP1 antibodies. Similar to the two subunits of CABP1, these two molecules appear to be encoded by a single gene and are probably generated by the same splicing mechanism described above. The CABP1 and p34/31 genes probably arose by duplication. Disruption of the gene which encodes p34 and p31 demonstrated that these two polypeptides appear to play a role in both growth and development of D. discoideum.