This is the observation that all introns in DNA begin with the nucleotides of "GT" (guanine, thymine) and end with the nucleotides "AG" (adenine, guanine). When the DNA is transcribed into RNA, the introns are removed from the RNA by a mechanism which recognizes these beginning and ending nucleotides - in the RNA they would be "CU" (cytosine, uracil) and "AC" (adenine, cytosine).