In 1956, Vernon M. Ingram, a protein chemist at the Cavendish Laboratory in Cambridge, developed a technique called fingerprinting, which maps on paper the various amino acids in a protein sample. Fingerprinting, a two-step process, utilizes paper electrophoresis and paper chromatography. It produces splotches on paper at various locations; each mark corresponds to a peptide residue (two or more linked amino acids). By comparing the fingerprints of sickle cell and normal hemoglobin, Ingram found that one spot differed between the two fingerprints and concluded that normal and sickle cell hemoglobin have a small difference in their amino acid sequences.

Less than one year later, Ingram conclusively showed that normal and sickle cell hemoglobin differ by one amino acid. Accordingly, he stated that normal hemoglobin has a glutamic acid in the fourth peptide chain whereas sickle cell hemoglobin has a valine at the same location. The substitution of glutamic acid with valine accounts for the difference in charges on the two types of hemoglobin, and for the observed electrophoretic difference. Ingram acknowledged the importance of the work accomplished by Pauling and his colleagues in 1949: “We owe to Pauling and his collaborators the realization that sickle cell anaemia is an example of an inherited ‘molecular disease’ and that it is due to an alteration in the structure of a large protein molecule, an alteration leading to a protein which is by all criteria still a haemoglobin. It is now clear that, per half-molecule of haemoglobin, this change consists in a replacement of only one of nearly 300 amino-acids, namely, glutamic acid, by another valine – a very small change indeed.”