Hemoglobin subunit delta is a protein that in humans is encoded by the HBD gene. == Function == The delta (HBD) and beta (HBB) genes are normally expressed in the adult: two alpha chains plus two beta chains constitute HbA, which in normal adult life comprises about 97% of the total hemoglobin. Two alpha chains plus two delta chains constitute HbA... Found on http://en.wikipedia.org/wiki/HBD