For those who have forgotten high school science, the numbers refer to the number of protons and neutrons in the carbon atom. All carbon atoms have six protons. Most also have 6 neutrons; that's carbon-12 or 12C. But some have seven neutrons; the 13C (carbon-13) isotope. Because a 13C atom is heavier than a 12C atom, some biochemical processes handle the different isotopes slightly differently, even though their fundamental chemistry is the same. This leads to a difference in the ratio of 12C to 13C between naturally-occurring human steroid hormones and the plant-derived precursors of synthetic steroids. - Ed.