Mouse lactate dehydrogenase-X (mLDH-X) was designed as a triple-resonance NMR reporter protein. The mLDH-X reporter catalyzes the conversion of a C-13-labeled pyruvic acid derivative (substrate) to a lactic acid derivative (product), which can be detected selectively by 1D H-1{C-13-C-13'} triple-resonance NMR. The expression of mLDH-X fused protein was successfully detected by monitoring the triple-resonance (HNMR)-H-1 signal of the product.