Ciliary function is essential for normal cellular activity in all species from simple protozoa upwards. In humans, ciliary dysmotility or complete immobility have been identified in autosomal recessive multisystemic diseases characterized by recurrent respiratory tract infections and male subfertility due to impaired sperm mobility. Linkage to human chromosome 19q13.3 has been published for some families but no candidate genes have been identified. We report the first identification of a mammalian homolog of a radial spokehead-like protein, with high homology to proteins of sea urchins and the protozoan Chlamydomonas reinhardtii, at the myotonic dystrophy-l locus (chromosome19q13.3). In the lower organisms, these proteins are important in normal ciliary or flagellar action, including that of sea urchin spermatozoa. Expression of the mammalian homolog was detected in the adult testis. We suggest that this gene, which we have called Radial Spokehead-Like 1 (RSHL1), is a candidate gene for familial primary ciliary dyskinesia.