SPP1 and injury: In the neonatal brain, OPN is the most highly expressed gene in a postnatal day 9 (PND9) hypoxia-ischemia (HI)-induced neonatal brain injury mouse model [8], and strong OPN immunoreactivity has been seen in axons at the periphery of the ischemic zone in both subacute and chronic brain injury lesions in human newborn infants [9].