Speaking of hemoglobin, I find it amazing that newborn babies have a different type of hemoglobin, fetal hemoglobin, that will be completely replaced by the adult form within a few weeks of birth. The fetal hemoglobin binds oxygen more readily than the adult form, thereby allowing the developing fetus to obtain access to oxygen from the mother's bloodstream via the placenta. The reactivation of fetal hemoglobin synthesis in adults has been used to treat sickle-cell disease since 1995.
When fetal hemoglobin production is switched off after birth, normal children begin producing hemoglobin A. But children with sickle-cell disease instead begin producing a long, slender form of hemoglobin called hemoglobin S. This variety of hemoglobin causes red blood cells to change their shape from round to sickle-shaped, which have a greater tendency to stack on top of one another and crowd blood vessels causing a variety of problems. If fetal hemoglobin remains the predominant form of hemoglobin after birth, however, these problems can be reduced.
Nature Medicine covered the breakthrough discovery of fetal hemoglobin promoting drugs such as hydroxyurea in 1995: Sickle cell paths converge on hydroxyurea.