Novel polypeptides, polynucleotide sequences, DNA constructs and compositions are provided for the preparation and use of polypeptides associated with naturally occurring polypeptides found in brains. The low molecular weight polypeptides either are growth inhibitors for neoplastic cells without inhibiting normal cells or affect GABA-ergic transmission. The polypeptides find use in inhibiting neoplastic growth, modulating diazepam receptor response, and detecting receptors for the polypeptides. Antibodies are provided in conjunction with the polypeptides, which may be used together or separately for detecting the presence of the polypeptides.