In a study published online at Science Express on 2 December 2010, Felisa Wolfe-Simon and colleagues described a bacterium from Mono Lake, California, which they claimed substitutes arsenic for a small amount of the phosphorus in its DNA and other molecules. If true, this finding would raise important questions about life’s basic requirements, since only six elements—carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus—make up the bulk of living matter.