The Csy4(H29A) entry in FlyBase represents an inactive form of the Pseudomonas aeruginosa Csy4 endoribonuclease (UniProtKB:Q02MM2). 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. Csy4 selectively recognizes a 16-nt hairpin sequence (represented by Csy4-HP in FlyBase) with exceptionally high affinity. 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).