Geminin also known as GMNN is a protein that plays a crucial role in regulating DNA replication initiation during the cell cycle, ensuring DNA replicates only once per cycle to maintain genomic stability. Geminin expression levels are tightly regulated, peaking during the S, G2, and M phases of the cell cycle, while remaining lowest during the G1 phase. This fluctuation is essential, as geminin degradation during mitosis, mediated by the anaphase-promoting complex (APC), allows for the re-establishment of the replication machinery in the subsequent cell cycle. Precise control of geminin levels is critical for cellular function, as dysregulation can lead to genomic instability and contribute to oncogenesis.