HEXA and Tay-Sachs disease: Tay Sachs disease, a severe neurodegenrative disease due to a number of different mutations described in the HEXA gene which encodes the alpha-subunit of the lysosomal enzyme beta-N-acetylhexosaminidase, has been proposed to owe its surprisingly high frequency to the benefit provided by protection from tuberculosis during the historical process of urbanization [57,58].