Maspardin is a 308 amino acid cytoplasmic protein that is widely expressed. Belonging to the AB hydrolase superfamily, Maspardin colocalizes with CD4 on endosomal/trans-Golgi network. It is thought that Maspardin may act as a negative regulatory factor in CD4-dependent T-cell activation. Defects in the gene encoding Maspardin are the result of hereditary spastic paraplegia autosomal recessive type 21 (also designated Mast syndrome), an autosomal recessive neurodegenerative disorder characterized by a slow, gradual, progressive weakness and spasticity of the lower limbs. The gene encoding Maspardin is encoded by human chromosome 15, which houses over 700 genes and comprises nearly 3% of the human genome.