D2 Model (IMAGE) University of Chicago Medical Center Caption In as many as 20 percent of people who rely on levothyroxine, a tiny genetic flaw in D2 causes the shuttling process to go astray. Those patients have a single-nucleotide substitution. As a result, one amino acid, threonine, is replaced by a different amino acid, alanine. Credit Antonio Bianco, MD, PhD, Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.