15-Deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) affects gene transcription by activating PPARgamma and by covalent addition to transcription factors and signaling molecules, However, it is not known whether the high concentrations of 15d-PGJ2 required for these responses are consistent with physiological levels. A new study suggests that in vivo 15d-PGJ2 levels are actually several orders of magnitude below the levels required to induce many of the biological effects attributed to this molecule.