In situ hybridization and preparation of digoxigeninlabeled riboprobes were performed as previously detailed (18, 19) . Antisense riboprobes were used to detect mRNA transcripts for proteolipid protein (PLP; gift from Dr. Lynn Hudson; National Institutes of Health [20] ) and PDGF>R (gift from Dr. Bill Richardson; University College London [21] ). The digoxigenin-labeled riboprobes were hybridized to 15-Km-thick coronal brain sections. Digoxigenin was detected with an alkaline phosphatase-conjugated sheep antidigoxigenin antibody (Boehringer Mannheim, Indianapolis, IN) followed by reaction with NBT/BCIP substrate (DAKO, Carpinteria, CA).