PNAS Classics are landmark papers from the archives of PNAS—significant scientific reports that have withstood the test of time. The authors of PNAS Classics opened new avenues of research within their respective fields. These groundbreaking discoveries offer insights into how the best science is practiced. Each PNAS Classic article is accompanied by a Classic Perspective or Commentary written by a scientist in the field. In some cases a Classic Profile written by a science writer is also available. Together these newly commissioned articles recall the impact of these discoveries, describe how they have advanced the field, and set them in current scientific context.