To determine whether the defective G2/M transition also occurs in vivo, we analyzed sarcomas arising in Mdm2la/la:Trp53R/R or Mdm2+/+:Trp53R/R mice by immunostaining for pH3(S10) to mark G2/M phase cells [26] or Ki67 to mark proliferating cells. This evidence concerns the gene MDM2 and sarcoma.