Insulin-like growth factor I receptor (IGF-IR) has been implicated in the normal and malignant growth of many cell types including cells from the central nervous system. In the cerebellar cortex IGF-IR mRNA is found in granular cells and IGF-I stimulation is mitogenic and protects cells from low-potassium-induced apoptosis. Since primitive neuroectodermal tumors/medulloblastomas (PNETs/medulloblastomas) are suspected to originate from the external cerebellar granular layer, it is reasonable to postulate that IGF-IR and/or its signaling molecules may contribute to the transformation of these poorly differentiated cells. To study activation of the IGF-IR system in medulloblastomas, we have utilized an antibody (anti-pY1316) that specifically recognizes the phosphorylated (active) form of the IGF-IR. Medulloblastoma biopsy specimens were positive when examined immunohistochemically with anti-Y1316 antibody. Further analysis of the IGF-IR system was performed in three human (Daoy, TE-671, D283 Med) and four mouse (BsB8, BsB13, Bs-1b, Bs-1c) medulloblastoma cell lines. All the murine cell lines examined express IGF-IR and PI3-kinase at relatively normal levels, and grossly overexpress IRS-1, when compared with normal mouse cerebellum. Within 15 min following IGF-I stimulation both mouse and human cell lines phosphorylate the beta subunit of the IGF-IR, IRS-1, Akt, and MAP kinases. They respond with cell proliferation when stimulated solely with IGF-I and are strongly inhibited when challenged with a dominant negative mutant of the IGF-IR (486/STOP), or with antisense oligonucleotides against the IGF-IR mRNA.