Spastic paraplegia 21(SPG21), also known as Maspardin, binds to the hydrophobic C-terminal amino acids of CD4 and may play a role as a negative regulatory factor in CD4-depenent T-cell activation. This protein is widely expressed in various tissues including heart, brain, placenta, lung, liver, skeletal muscle, kidney and pancreas. Mutations in SPG21 cause Mast syndrome, an autosomal-recessive complicated form of hereditary spastic paraplegia characterized by dementia, thin corpus callosum and white matter abnormalities. Recombinant human SPG21 protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.