I-CreI is a homing endonuclease whose gene was first discovered in the chloroplast genome of Chlamydomonas reinhardtii, a species of unicellular green algae.

I-CreI enzyme, which functions as a homodimer, recognizes a 22-nucleotide sequence of duplex DNA and cleaves one phosphodiester bond on each strand at specific positions.

A 1991 study established that the ORF codes for a DNA endonuclease, I-CreI, which selectively cuts a site corresponding to where the intron is spliced out of the 23S primary transcript.

[2] I-CreI has evolved to cut a 22-nucleotide sequence of DNA that occurs in alleles of the 23S ribosomal RNA gene that lack the I-CreI-containing intron.

[7][8][9] A genetic system in Saccharomyces cerevisiae has also been created, yielding additional I-CreI mutants with modified homing site specificities.

[12] It seems very likely that advances in molecular biological techniques and generation of a library of I-CreI-derived novel endonucleases will eventually allow for the targeting of many genes of etiological significance.