CDKN1B and medulloblastoma: p27Kip1 loss in mice can increase tumor formation [4], and p27Kip1 haploinsufficiency has been further supported by studies using the Patched1 [GenBank: NM_008957] heterozygous (Ptc1+/−) and SmoA1 mouse medulloblastoma models [5,14], where Shh-induced medulloblastoma incidence was accelerated by loss of one or both p27Kip1 alleles.