X-ray diffraction and electron microscope studies of hard keratins (e.g. claws, scales, feathers and hair) have shown that they all have a filamentous texture but that the molecular structure of the filaments in mammalian keratins is quite different from that in avian keratins. The framework of the filaments in mammalian keratin consists of two-strand coiled coils of alpha-helices whereas the framework in avian keratins is composed of beta-sheets. Reptilian hard keratins have not been studied in detail but the X-ray diffraction pattern is very similar to that obtained from avian hard keratins leading to the supposition that the framework of the filaments is also composed of beta-sheets. The present contribution describes an analysis of the sequence of a lizard claw protein using structural probes which reveal the origins of the common structural features of the filaments in avian and reptilian keratin.