CCND1 and neoplasm: However, as we have previously shown that Btg1 inhibits selectively cyclin D1 (12), we observe here that the ablation of Btg1 is associated to an increase of cyclin D1 expression (though attaining statistical significance only for the transcript) in MB tumor cells, indicating that the proliferative system is activated, despite the lack of increased proliferation of Ptch1+/−/Btg1KO GCPs or tumor cells.