– Phosphoserine phosphatase (PSP) is an enzyme that belongs to the serB family. PSPH catalyzes magnesium-dependent hydrolysis of L-phosphoserine and is also involved in an exchange reaction between L-serine and L-phosphoserine. The reaction mechanism proceeds via the formation of a phosphoryl-enzyme intermediates. Deficiency of this protein is thought to be linked to Williams syndrome. A disorder that results in pre- and postnatal growth retardation, moderate psychomotor retardation and facial features suggestive of Williams syndrome.