Either of two oncogenes (proto-cancer gene) found in the avian erythroblastosis retrovirus (a retrovirus which affects red blood cell formation in birds). Erb-a is incredibly similar to a gene which codes for the receptor protein that recognizes hormones from the thyroid gland for the cell.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.