HNCA is a 3D triple-resonance NMR experiment commonly used in the field of protein NMR. The name derives from the experiment's magnetization transfer pathway: The magnetization of the amide proton of an amino acid residue is transferred to the amide nitrogen, and then to the alpha carbons of both the starting residue and the previous residue in the protein's amino acid sequence. In contrast, the complementary HNCOCA experiment transfers magnetization only to the alpha carbon of the previous residue. The HNCA experiment is used, often in tandem with HNCOCA, to assign alpha carbon resonance signals to specific residues in the protein. This experiment requires a purified sample of protein prepared with C and N isotopic labelling, at a concentration greater than 0.1 mM, and is thus generally only applied to recombinant proteins.