It stimulates production of another protein that complexes with cdk2 to halt cell division.  
This protein is highly multifunctional and consists of a transactivation domain, a proline rich domain, a DNA binding domain, and a C-terminal regulatory region.  Somatic mutations in this protein can be accompanied by the loss of the short arm of chromosome 17, while a large fraction of sufferers of Li-Fraumeni (lee-froh-may-nee) syndrome have a germline mutation in the gene coding for this protein. It stimulates production of another protein that complexes with cdk2 to halt cell division.  The wild-type allele is a tumor suppressor, while some mutant varieties are oncogenes.  For 10 points - name this protein, abnormal forms of which are implicated in about half of all human cancers.
This protein is highly multifunctional and consists of a transactivation domain, a proline rich domain, a DNA binding domain, and a C-terminal regulatory region.  
p53