Small GTP-binding proteins consist of one single, 21 kD polypeptide chain. These regulatory proteins are active in the GTP-bound but inactive in the GDP-bound form. They are involved in the regulation of several important cell functions, such as cell division, cell motility, shape changes, intracellular membrane traffic, transport of macromolecules between cytoplasm and nucleus, assembly of enzyme complexes. Recently several mutations have been described which modify the function of small G-proteins resulting in typical pathological changes. These diseases and the molecular alterations involved in their pathomechanism are summarized in this review.