Members of the box H/ACA family contain an ACA triplet, exactly 3 nt upstream from the 3' end and an H-box in a hinge region that links two structurally similar functional domains of the molecule. Both boxes are important for snoRNA biosynthesis and function. A few box H/ACA snoRNAs are involved in rRNA processing; most others are known or predicted to participate in selection of uridine nucleosides in rRNA to be converted to pseudouridines. Site selection is mediated by direct base pairing of the snoRNA with rRNA through one or both targeting domains.