Neurotrypsin (NT), a neuronal trypsin-like serine protease, has been recognized to play an essential role in cognitive brain function due to a 4-nucleotide deletion in the PRSS12 gene of the human chromosome 4q, which results in an earlier stop codon and the production of a truncated protein leading to severe mental retardation in humans [3]. Here, PRSS12 is linked to Intellectual disability.