PTPN20 and Aqueductal stenosis: However, Ptpn20−/− mice did not exhibit severe hydrocephalus after birth like H-Tx rats, and we hypothesize that this may be because Ptpn20−/− mice did not develop aqueductal stenosis during development or that the overproduction of CSF caused by NKCC1 in the genetic background of C57BL/6J was insufficient to cause severe hydrocephalus.