Membrane-associated tyrosine- and threonine-specific cdc2-inhibitory kinase also known as Myt1 kinase is an enzyme that in humans is encoded by the PKMYT1 gene. Myt 1 is an enzyme in the Wee1 family and found in vertebrates. The Wee 1 family includes a variety of enzymes that all work to inhibit Cdk activity in a variety of different organisms. Myt 1 is important in regulating the cell cycle through inactivating Cdks through the phosphorylation of both Tyr 15 and Thr 14 Wee 1 family There are other enzymes that play roles in the inactivation of Cdks in the Wee 1 family that work in a variety of different organisms. Some examples of Wee 1 family enzymes working in different species are as follows: S. Cerevisiae: Swe 1 S. Pombe: Wee 1 and Mik 1 D. Melanogaster: Dwee 1 and Dmyt 1 Vertebrates: Wee 1 (phosphorylates only Tyr 15) and Myt 1 (phosphorylates both Tyr 15 and Thr 14) Wee 1 is present in all eukaryotes under different names and is responsible for phosphorylating Tyr 15 in each of those organisms. In fission yeast where Wee 1 is the primary inhibitor of Cdk1, mutations in the Wee 1 gene cause premature entry into mitosis. On the other hand, the overproduction of Wee 1 blocks entry into mitosis. Tyrosine phosphorylation Myt 1 plays an important role in inhibiting Cdk activity through phosphorylating Tyr 15 and Thr 14. Tyr 15 is highly conserved and is found in all major Cdks, however has different names in different organisms. Animal cells have an additional Thr 14 site that aids in further Cdk inactivation. Tyr 15 and Thr 14 are located on Cdks at their ATP binding site. It is thought that the phosphorylation of Tyr 15 and Thr 14 interferes with the orientation of ATP phosphates which inhibits Cdk functioning. The phosphorylation of these sites is particularly important during the beginning of mitosis as they are involved in the activation of M-Cdks. They are also believed to be involved in the timing of activation of S-phase Cdks and the entry into G1/S phase. With Myt 1 inactivating Cdks by phosphorylating both Tyr 15 and Thr 14, there needs to be a method of dephosphorylating the sites so that the Cdk can become active once again. This dephosphorylation of inhibitory sites is done by the Cdc25 family. In vertebrates, the Cdc25 enzymes are Cdc25A which controls the