Homeobo, Xenopus/Amphibian, protein Ho, Xenopus/Amphibian,-A1 (Ho, Xenopus/Amphibian,-A1) is a sequence-specific transcription factor which is part of a developmental regulatory system that provides cells with specific positional identities on the anterior-posterior a, Xenopus/Amphibian,is. Ho, Xenopus/Amphibian,-A1 acts on the anterior body structures and on the maintenance and/or generation of hindbrain segments. Defects in HO, Xenopus/Amphibian,A1 are the cause of Athabaskan brainstem dysgenesis syndrome (ABSD), also known as Narvajo brainstem syndrome. This syndrome is characterized by horizontal gaze palsy, sensorineural deafness, central hypoventilation, and developmental delay. Some patients have swallowing dysfunction, vocal cord paralysis, facial paresis, seizures, and cardiac outflow tract anomalies.