The CAD protein, also known as carbamoyl phosphate synthetase II, is a multifunctional enzyme that plays a crucial role in the initiation of de novo pyrimidine biosynthesis in mammalian cells. It is a key regulatory enzyme involved in the synthesis of pyrimidines, which are essential components of nucleic acids. The CAD protein is composed of multiple domains that catalyze the first three steps of pyrimidine synthesis: carbamoyl phosphate synthesis, dihydroorotase, and aspartate transcarbamoylase activities. The allosteric regulation of CAD modulates de novo pyrimidine synthesis during the cell cycle, and its activity is influenced by various effectors and subunit interactions. Moreover, the CAD protein has been implicated in coronary artery disease (CAD) through the integration of GWAS data and human plasma proteomes.