FAM91A1 is a component of the WDR11 complex that functions downstream of the AP-1 clathrin coat to mediate vesicle tethering and cargo protein transport 1. The WDR11-FAM91A1 complex recognizes acidic-cluster-containing cargo proteins in a sequence-specific manner, facilitating endosome-to-trans-Golgi network (TGN) trafficking 1. FAM91A1 directly interacts with TBC1D23 to regulate endosomal trafficking of KIAA0319L, a protein essential for axonal growth and neuronal development 2. Mutations in FAM91A1 or its binding partner WDR11 are associated with pontocerebellar hypoplasia (PCH), a neurodevelopmental disorder, suggesting that disrupted endosomal trafficking underlies this condition 2. Beyond its canonical trafficking function, FAM91A1 expression is dysregulated in certain cancers; high expression correlates with poor prognosis in osteosarcoma 3, and FAM91A1 has been identified as part of oncogenic fusion events in breast cancer 4. Additionally, FAM91A1 expression is regulated by microRNA-mediated mechanisms in embryonic stem cells, where it participates in pluripotency maintenance 5.