Cripto, also known as teratocarcinoma derived growth factor 1 (TDGF-1), belongs to the EGF-CFC family of proteins. Members of this family are characterized by an N-terminal signal peptide, a conserved cysteine rich domain (CFC motif), and a short hydrophobic carboxy-terminal tail that contains GPI cleavage and attachment sites. The GPI moiety anchors Cripto and family members to the extracellular plasma membrane (1). An O-linked fucosylation site within the EGF-like motif is required for Cripto and related family members to perform their function as co-receptors for TGF-beta-related ligands such as Nodal and Vg1/GDF1 (2,3). Soluble forms of Cripto can be produced - these contain intact EGF and CFC domains, and are thought to have paracrine activities, as opposed to the autocrine activity of Cripto functioning as a coreceptor (4). Understanding of this paracrine activity is not complete, but it is proposed that Cripto may act as co-ligand for Nodal (3).Cripto is an important modulator of embryogenesis and oncogenesis (4). It is highly expressed in early embryos, and in embryonic stem (ES) cells where it is involved in cardiomyocytic differentiation and acts as a negative regulator of neurogenesis (5-7). Transient activation of Cripto is essential for the capacity of stem cell self-renewal and pluripotency in ES cells, and in some adult derived stem cells (8). Signaling through Cripto can also stimulate other activities that promote tumorigenesis such as stimulation of proliferation, cell motility, invasion, angiogenesis and epithelial-mesenchymal transition (EMT) (9-11). Cripto is highly expressed in a broad range of tumors, where it acts as a potent oncogene.