A family of proteins called cyclins helps cells move throughthe cell cycle. Cyclins and their partner, cyclin-dependentkinase (Cdk), form a complex that triggers the proteinkinase activity of the latter. Cyclins get their name from thefact that they are made or broken down as necessary topropel cells through the many phases of the cell cycle.Cyclin separates from CDK at low cell concentrations,preventing the enzyme from functioning, most likely byblocking the catalytic site with a protein chain. One of theimportant cell-cycle regulators is the cyclin-D1 antibody,also known as PRAD-1 or bcl-1, which phosphorylates theRb protein in conjunction with Cdk4 and/or Cdk6. Manydifferent types of human neoplasms, including Mantle CellLymphomas, have an overexpression of this suspectedproto-oncogene. Breast cancer has been shown tooverexpress cyclin-D1.