We also examined by immunohistochemistry the expression of IRS1, Akt/Pkb and Erk1/2 kinases, downstream mediators of the IGF-I signaling pathway, in medulloblastoma samples from Ptc1+/- and Ptc1+/-/IGF-I Tg mice (n = 3). Here, IRS1 is linked to medulloblastoma.