The IDGF's (imaginal disc growth factors) are a family of growth factors identified in insects that include at least five members, some of which are encoded by genes in a tight cluster. The IDGF's have an eight-stranded alpha/beta barrel fold and are related to the glycosyl hydrolase family 18 (GH18) chitinases, but they have an amino acid substitution known to abolish chitinase catalytic activity. IDGFs may have evolved from chitinases to gain new functions as growth factors, interacting with cell surface glycoproteins involved in growth-promoting processes.