AbstractMüllerian inhibiting substance (MIS or anti-Müllerian hormone) is a member of the transforming growth factor-β family and plays a pivotal role in proper male sexual differentiation. Members of this family signal by the assembly of two related serine/threonine kinase receptors, referred to as type I or type II receptors, and downstream cytoplasmic Smad effector proteins. Although the MIS type II receptor (MISRII) has been identified, the identity of the type I receptor is unclear. Here we report that MIS activates a bone morphogenetic protein-like signaling pathway, which is solely dependent on the presence of the MISRII and bioactive MIS ligand. Among the multiple type I candidates tested, only ALK2 resulted in significant enhancement of the MIS signaling response. Furthermore, dominant-negative and antisense strategies showed that ALK2 is essential for MIS-induced signaling in two independent assays, the cellular Tlx-2 reporter gene assay and the Müllerian duct regression organ culture assay. In contrast, ALK6, the other candidate MIS type I receptor, was not required. Expression analyses revealed that ALK2 is present in all MIS target tissues including the mesenchyme surrounding the epithelial Müllerian duct. Collectively, we conclude that MIS employs a bone morphogenetic protein-like signaling pathway and uses ALK2 as its type I receptor. The use of this ubiquitously expressed type I receptor underscores the role of the MIS ligand and the MIS type II receptor in establishing the specificity of the MIS signaling cascade.