OAS is caused by homozygous pathogenic variants in HMX1 (H6 Family Homeobox 1), encoding a homeobox transcription factor with high affinity toward a consensus HMX-binding site harboring a 5′-CAAGTG-3′ element required for sensory organ development [65,66,67,68]. Here, HMX1 is linked to microphthalmia with limb anomalies.