The Csy4-HP entry in FlyBase represents a 16-nt hairpin sequence which is selectively recognized with exceptionally high affinity by the Pseudomonas aeruginosa Csy4 endoribonuclease. Wild type Csy4 is an endoribonuclease that participates in the P. aeruginosa UCBPP-PA14 CRISPR system by cleaving transcripts to liberate guide RNAs used for adaptive immunity against foreign DNA. The active site of Csy4 contains an essential histidine residue (H29) - mutation of H29 to alanine (H29A) inactivates Csy4 without affecting substrate binding affinity or specificity (PMID:23493562). If a protein of interest is tagged with Csy4(H29A), then that protein can be irreversibly bound to a copy of Csy4-HP that has been inserted into the genome (e.g. FBrf0247286).