Before revision: When the I-CreI-containing intron encounters a 23S gene lacking the intron, I-CreI enzyme "homes" in on the "intron-minus" allele of 23S and effects its parent intron's insertion into the intron-minus allele.
After revision: When the I-CreI-containing intron encounters a 23S allele lacking the intron, I-CreI enzyme "homes" in on the "intron-minus" allele of 23S and effects its parent intron's insertion into the intron-minus allele.