A comprehensive curation demonstrates the dynamic evolutionary patterns of prokaryotic CRISPRs

SpacerDB consists of all the annotated CRISPR spacer sequences in the database CRISPRdb, and was downloaded from the web site of CRISPRdb (Grissa, et al., 2007) on April 14, 2014. CRISPR spacers are not random nucleotide sequences, and are supposed to originate from the foreign invasive elements like phages (Deveau, et al., 2010). So a DR flanking sequence matching a known spacer may also be a spacer.