The metabolism of cancer cells is characterized by an increased rate in the production of nucleotides, which fuel the DNA and RNA synthesis during rapid cell growth. The de novo biosynthesis of pyrimidines is initiated and controlled by CAD, a 240 kDa mulifunctional polypeptide that self-assembles into large hexameric particles of ~1.5 MegaDa. The up-regulation of CAD during cell proliferation and its phosphorylation through different signalling cascades has awakened the interest in its potential use as a target for anticancer therapy. Our research aims to determine the structure of CAD to understand its catalytic and regulatory mechanisms. I will report recent structural and biochemical characterization of the DHO and ATC domains of human CAD and of their association to form the basic scaffold of CAD hexameric particles.