This study provides the first in vivo and in vitro demonstration that: (1) PACAP suppresses stathmin1 expression; (2) increased expression of stathmin1 in SGZ neurons, due to PACAP deficiency, causes abnormal axonal sprouting and stathmin1 over-expression in primary cultured hippocampal neurons induces abnormal arborization; (3) ascl1 regulates stathmin1 expression; (4) up-regulation of Id3 and Hes1 by PACAP stimulation inhibits Ascl1; and (5) stathmin1 expression was increased in the brains of patients with schizophrenia. The gene discussed is ID3; the disease is schizophrenia.