THE protein Sonic hedgehog (Shh) is essential for a variety of patterning events during development. It is the signal from the notochord that induces ventral cell fate in the neural tube and somites(1,2), and is the polarizing signal for patterning of the anterior-posterior axis of the developing limb bud(3). Because of these and other inductive functions of Shh, it is important to understand how the Hedgehog (Hh) signal is received by the target cells. Here we describe binding studies using labelled Shh that strongly suggest that the Hh receptor is encoded by patched (ptc), a gene first identified in genetic screens in Drosophila(4).