Component of a complex required to localize phosphatidylinositol 4-kinase (PI4K) to the plasma membrane. The complex acts as a regulator of phosphatidylinositol 4-phosphate (PtdIns(4)P) synthesis. FAM126A plays a key role in oligodendrocytes formation, a cell type with expanded plasma membrane that requires generation of PtdIns(4)P. Its role in oligodendrocytes formation probably explains its importance in myelination of the central and peripheral nervous system. May also have a role in the beta-catenin/Lef signaling pathway. Component of a phosphatidylinositol 4-kinase (PI4K) complex, composed of PI4KA, EFR3 (EFR3A or EFR3B), TTC7 (TTC7A or TTC7B) and FAM126 (FAM126A or FAM126B). Interacts with TTC7 (TTC7A or TTC7B), interaction is direct. Localizes to the cytosol and is recruited to the plasma membrane following interaction with other components of the phosphatidylinositol 4-kinase (PI4K) complex. Predominantly expressed in the central nervous system, where it is found in neurons but not in myelinating cells. Lower abundance is observed in peripheral neurons, where it is detectable only at early postnatal ages (PubMed:22461884). Expressed in both oligodendrocytes and neurons (PubMed:26571211). Down-regulated by beta-catenin. No visible phenotype. Lysates from total brain and optic nerve (a pure white matter tract) display a selective decrease of Ttc7a and Efr3a protein levels. Belongs to the FAM126 family.