The PIWI family of proteins is known to be important for germ cell development. PIWI proteins were recently discovered to bind thousands of small RNAs, termed piRNAs (Aravin et al., 2006; Girard et al., 2006; and Lau et al., 2006: Characterization of the piRNA complex from rat testes. Science 313, 363-367). piRNAs have been identified in testis, are 26-31 nucleotide in length, and are distinct from miRNAs. Over 176,000 piRNA candidates have been cloned and sequenced from mouse, human and rat, and are included for the first time in the current release of RNAdb.