So radioactively labeled DNA molecules. And the way that they did that is they took a medium that was very high, very rich in isotopic nitrogen atoms. So m 15. Now, remember, the normal nitrogen atom contains an atomic mass of 14, and that's because it contains seven protons and seven neutrons in the nucleus. But in the case of this heavy isotopic nitrogen atom, we have eight neutrons and seven protons. And so eight plus seven gives us an atomic mass of 15.