HEG1 (HEG homolog 1) is also known as HEG and is a 1,381 amino acid protein that exists as two alternatively spliced isoforms, one of which is a single-pass membrane protein and the other of which is secreted. HEG1, which is expressed in endothelial cells and smooth muscle cells of heart tissue, contains two EGF-like domains that play a role in calcium-binding events which may regulate concentric heart growth. The gene encoding HEG1 maps to human chromosome 3, which houses over 1,100 genes, including a chemokine receptor (CKR) gene cluster and a variety of human cancer-related gene loci. Key tumor suppressing genes on chromosome 3 include those that encode the apoptosis mediator RASSF1, the cell migration regulator HYAL1 and the angiogenesis suppressor SEMA3B. Marfan Syndrome, porphyria, von Hippel-Lindau syndrome, osteogenesis imperfecta and Charcot-Marie-Tooth Disease are a few of the numerous genetic diseases associated with chromosome 3.