For simplicity one can think of the + class as having one extra base at some point or other in the genetic message and the - class as having one too few.
Francis CrickJim and I hit it off immediately, partly because our interests were astonishingly similar and partly, I suspect, because a certain youthful arrogance, a ruthlessness, an impatience with sloppy thinking can naturally to both of us.
Francis Crick