As inhibition of MMP9 blocked the aberrant cell proliferation and re-established tissue polarity [62], it is possible that the observed down-regulation of MMP9 is not only a consequence but contributes to the significant reduction in malignant progression of PNLs to medulloblastomas in Ptch1Δ/+Pn-1Δ/+ mice (this study). This evidence concerns the gene MMP9 and medulloblastoma.