AP1B1 encodes the β1 subunit of the heterotetrameric adaptor protein 1 (AP-1) complex, a clathrin-associated protein essential for vesicular trafficking and protein sorting 1. The AP-1 complex mediates recruitment of clathrin to membranes and recognition of sorting signals in transmembrane cargo, functioning primarily in the trans-Golgi network and endosomes 2. Loss of AP1B1 function destabilizes the entire AP-1 complex; affected cells show reduced γ subunit levels, abnormal vesicle accumulation, and disrupted epidermal differentiation and junction proteins 1. Pathogenic biallelic variants in AP1B1 cause autosomal recessive Keratitis-Ichthyosis-Deafness syndrome (KIDAR), a rare multisystem disorder 3. KIDAR patients present with ichthyosis, palmoplantar keratoderma, progressive sensorineural hearing loss, photophobia, keratitis, and failure to thrive 4. Additional features include enteropathy, developmental delay, thrombocytopenia, and abnormal copper metabolism, reflecting impaired trafficking of copper-transporting ATPases ATP7A and ATP7B 2. Notably, AP1B1 deletions also contribute to intellectual disability when occurring as larger chr22 microdeletions 5. With ten reported KIDAR cases to date, AP1B1 represents a critical regulator of cellular homeostasis whose dysfunction causes widespread epithelial and sensory dysfunction 2.