<p>Expansins are unusual proteins that mediate cell wall extension in plants [<cite idref="PUB00006126"/>]. They are believed to act as a sort of chemical grease, allowing polymers to slide past one another by disrupting non-covalent hydrogen bonds that hold many wall polymers to one another. This process is notdegradative and hence does not weaken the wall, which could otherwise rupture under internal pressure during growth.</p><p>Sequence comparisons indicate at least four distinct expansin cDNAs in rice and at least six in Arabidopsis. The proteins are highly conserved insize and sequence (75-95% amino acid sequence similarity between any pairwise comparison), and phylogenetic trees indicate that this multigenefamily formed before the evolutionary divergence of monocotyledons and dicotyledons [<cite idref="PUB00006126"/>]. Sequence and motif analyses show no similarities to known functional domains that might account for expansin action on wall extension. It is thought that several highly-conserved tryptophans may function in expansin binding to cellulose, or other glycans. The high conservation of the family indicates that the mechanism by which expansins promote wall extensin tolerates little variation in protein structure. </p><p>Grass pollens, such as pollen from timothy grass, represent a major cause of type I allergy [<cite idref="PUB00007280"/>]. Interestingly, expansins share a high degree ofsequence similarity with the Lol p I family of allergens.</p>