AN electrophoretically fast genetic variant of adult haemoglobin was described in six members of a Negro family living in Texas by Thompson et al. in 1963 (ref. 1). Fingerprints of the purified haemoglobin established the identity of this variant with haemoglobin I previously described by Murayama and Ingram2, in which the third tryptic peptide from the N-terminus of the α chain contained an amino-acid alteration responsible for the rapid electrophoretic migration of the intact haemoglobin. Lysine, which is the sixteenth residue, had been replaced in haemoglobin I by an acidic amino-acid. The substitution was described in both I variants from Texas1 and Philadelphia3 as being lysine to aspartic acid; however, recent elucidation of the genetic code has eliminated the substitution lys→asp from the class of mutations which could result from alteration of a single nucleotide base in the triplet code4. A nucleotide triplet coding for lysine (AAA or AAG) could not be altered in a single base so that it would specify aspartic acid (GAU or GAC). This led Beale and Lehmann5 to re-examine the I variant from Philadelphia. Their findings established the substitution to be lys→glu, which is in agreement with an alteration in a single nucleotide (AAA or AAG to GAA or GAG).