Model for CD26 regulation of adhesion and downstream signaling. In this simplified scheme, CD26 is shown bound to the cell membrane. Versican is also depicted in the membrane, but is also secreted and is a constituent of the extracellular matrix. CD44 and HA are bound to versican, but CD44 is also bound to MT1-MMP, which can itself cleave CD44, resulting in CD44 secretion. Secretion of the cleaved CD44 is necessary for localization of MT1-MMP at the invadopodia where it digests collagen I, a constituent of the extracellular matrix. In addition, Erk (1/2) activation occurs in the parental Karpas cells and has been reported to be required for migration, invasion, and CD44 upregulation. This model is intended to be a working hypothesis of the relationship between the proteins shown here.