Every cell that's ever lived has been the result of the four-letter genetic alphabet: A, T, C and G -- the basic units of DNA. But now that's changed. In a visionary talk, synthetic biologist Floyd E. Romesberg introduces us to the first living organisms created with six-letter DNA -- the four natural letters plus two new man-made ones, X and Y -- and explores how this breakthrough could challenge our basic understanding of nature's design.