Two new NMR structures of WW domains, the mouse formin binding protein and a putative 84.5 kDa protein from Saccharomyces cerevisiae, show that this domain, only 35 amino acids in length, defines the smallest monomeric triple-stranded antiparallel beta-sheet protein domain that is stable in the absence of disulfide bonds, tightly bound ions or ligands. The structural roles of conserved residues have been studied using site-directed mutagenesis of both wild type domains. Crucial interactions responsible for the stability of the WW structure have been identified. Based on a network of highly conserved long range interactions across the beta-sheet structure that supports the WW fold and on a systematic analysis of conserved residues in the WW family, we have designed a folded prototype WW sequence.