Novel receptor polypeptides, polynucleotides encoding the polypeptides, and related compositions and methods are disclosed. The polypeptides comprise an extracellular domain of a cell-surface receptor that is expressed in testis cells. The polypeptides may be used within methods for detecting ligands that promote the proliferation and/or differentiation of testis cells, and may also be used in the development of male-specific contraceptives and infertility treatments.