Plays a role in ciliogenesis. The disease is caused by variants affecting the gene represented in this entry. Defects in HYLS1 may be involved in ciliopathies other than hydrolethalus syndrome 1. A homozygous mutation resulting in a C-terminal extension of 11 residues has been found in patients diagnosed as Joubert syndrome, a ciliopathy presenting with cerebellar ataxia, oculomotor apraxia, hypotonia, neonatal breathing abnormalities and psychomotor delay. Neuroradiologically, it is characterized by cerebellar vermian hypoplasia/aplasia, thickened and reoriented superior cerebellar peduncles, and an abnormally large interpeduncular fossa, giving the appearance of a molar tooth on transaxial slices (molar tooth sign). Additional variable features include retinal dystrophy and renal disease. Belongs to the HYLS1 family.