The anaphase-promoting complex (APC) or cyclosome is a cell cycle-regulated ubiquitin-protein ligase that regulates important events in mitosis such as the initiation of anaphase and exit from telophase. The APC, in conjunction with other enzymes, assembles multi-ubiquitin chains on a variety of regulatory proteins thereby targeting them for proteolysis by the 26S proteasome. CDC26 is one of the nine or so subunits identified within APC but its exact function is not known. The APC/C becomes active at the metaphase/anaphase transition and remains active during G1 phase. One mechanism linked to activation of the APC/C is phosphorylation. The yeast APC/C is composed of at least 13 subunits, but the function of many of the subunits is unknown. Hcn1 is the smallest subunit of the S. pombe APC/C, and is found to be essential for cell viability, APC/C integrity, and proper APC/C regulation. In addition, Hcn1 phosphorylation indicates a specific role for the phosphorylation of this subunit late in the cell cycle.