Please suggest what type of mutation occurs in pku?

PKU is inherited by : autosomal recessive transmission. Both parents carry a mutation of a gene on the long arm of Chromosome 12, causing deficiency of the enzyme phenylalanine hydroxylase in a child who inherits both mutant genes. The child is unable to use an amino acid called Phenylanine. Excess Phenylalanine from dietary protein is toxic to the brain.See http://www.npkua.org/Education/AboutPKU.aspx