For its third in a series "honoring individuals who have made extraordinary contributions to the advancement of science," the US Postal Service has issued stamps that commemorate the achievements of the late American scientists Melvin Calvin, Asa Gray, Maria Goeppert Mayer, and Severo Ochoa. The federal mail-carrier says it is honoring the late biochemist and Nobel-laureate Ochoa for his pioneering work in RNA synthesis. "The stamp shows Ochoa in his laboratory in 1959, along with figures representing some of his work on protein synthesis," the USPS says.