Hh

Etymology 1

 * the gene and the signaling pathway are named after the protein.

Proper noun

 * 1)  A protein expressed in the larva of the fruit fly, required for the proper development of the larval body.
 * 2) The gene coding for the Hh (Hedgehog) protein.
 * 3) The signaling pathway involving the Hh (Hedgehog) protein, and its homologues in other animals.

Etymology 2
From the two antigenic variants involved in the blood group system (the H and h antigens).

Proper noun

 * 1)  A blood group system involving the presence of either the H or the h antigen on the surface of a person's red blood cells.