Abstract: Deficiencies in the enzymatic activity of acid sphingomyelinase (ASM) result in Niemann-Pick disease. A variety of modifications which eliminate the activity of the free thiol on the C-terminal cysteine residue of ASM all result in substantially increased specific activity of the enzyme. Methods used to alter the activity of this residue include site-directed mutagenesis to delete or alter the residue, enzymatic degradation of the ASM to remove the residue, copper-promoted dimerization of ASM (via the terminal cysteine residues) and chemical modification of the free thiol group on this residue.