Hypomyelinating leukodystrophies (HLD) are hereditary central nervous system diseases in which the myelin sheath does not form properly. The disease prototype is the X-linked recessive Pelizaeus-Merzbacher disease (also now known as HLD1), which is caused by the mutation, multiplication, or deletion of the plp1 gene. PLP1 missense mutations lead to protein aggregation and accumulation in subcellular compartments such as the endoplasmic reticulum (ER). The gene responsible for an autosomal recessive Pelizaeus-Merzbacher-like disease called HLD5 is named fam126a (also known as hyccin or drctnnb1a). While the gene mutations often cause FAM126A protein deficiency, one known missense mutation, Leu-53-to-Pro (L53P), allows some protein to be produced. Here, we show that the L53P mutant aggregates in cells, accumulating primarily in the ER. This is in contrast to the wild type FAM126A, which distributes throughout the cytoplasm. In addition, the L53P mutant expression promotes the activities of kinases involved in unfolded protein response. These results suggest that a disease-associated FAM126A missense mutation causes protein accumulation in subcellular compartments, possibly to mediate a disease-associated phenotype, which is similar to what is seen with PLP1.