Application of a differential hybridization technique led to the isolation of a human pituitary cDNA clone encoding the complete structure of the polypeptide 7B2. This protein of unknown function, which is sorted to secretory granules, appears to be present selectively in neurons and endocrine cells. The polypeptide chain of human 7B2, preceded by a cleaved signal peptide, comprises 185 amino acids (a calculated Mr of 20,793). Interesting features of the highly-conserved 7B2 structure include (i) a serine phosphorylation consensus sequence, (ii) the occurrence of three pairs of dibasic amino acids representing potential proteolytic cleavage sites and, in particular, (iii) the presence of three regions homologous to GTP-binding domains giving 7B2 structural characteristics of a GTP-binding protein.