The biologically active form of Hh molecules is obtained by autocatalytic splitting of their precursor proteins, and corresponds to approximately the N-terminal one half of the precursor molecule. Although Hh proteins have unique expression patterns and distinct biological roles within their respective regions of secretion, they use the same signaling pathway and can substitute for each other in experimental systems. Recombinant E. coli-derived Human Sonic HedgeHog is a 19 kDa protein consisting of 176 amino acid residues, including an N-terminal Ile-Val-Ile sequence substituted for the naturally occurring chemically modified Cys residue.