CIZ1 (Cip1 interacting zinc-finger protein) was identified as a protein that associates with the cell-cycle inhibiting protein, p21 (Cip1/Waf1). CIZ1 is a member of the matrin 3 protein family and has been shown to bind DNA consensus sequences and co-localize with PCNA during replication. In breast cancer cells, CIZ1 was shown to associate with Cdk2 and dynein light chain 1 (DLC1), a cytoskeletal signaling protein involved in cell cycle regulation and tumorigenesis. Studies of the role of CIZ1 in breast tumorigenesis revealed that CIZ1 is an estrogen-responsive gene that functions as a coactivator of the estrogen receptor. Targeted depletion of CIZ1 inhibits entry into S-phase. This finding along with evidence that CIZ1 interacts with multiple cell cycle regulatory proteins and localizes to DNA replication foci, indicates that CIZ1 plays a critical role in the regulation of DNA replication.