Features include always present findings: Hearing loss (hearing impairment); and very common findings: Full cheeks, Narrow mouth, and Question mark ear. 11 total HPO annotations.

HDAC9 encodes histone deacetylase 9 (1,011 aa). Responsible for the deacetylation of lysine residues on the N-terminal part of the core histones (H2A, H2B, H3 and H4). Highest expression in Artery Aorta (8.3 TPM) and Esophagus Muscularis (7.7 TPM).

Auriculocondylar syndrome 4 is associated with mutations in the HDAC9 gene on chromosome 7.

The HDAC9 protein participates in Notch-HLH transcription pathway pathway.

HDAC9 is classified as a druggable target (Drug Resistance, Druggable Genome, Enzyme, Transcription Factor, and Transcription Factor Complex categories) with score 2.4.

Genetic testing for HDAC9 is available. Testing is considered confirmatory for diagnosis.

Phenotype severity distribution: 1 always present feature, 3 very common features, 4 common features.