Sandhoff is caused by a mutation in the HEXB gene on chromosome 5. The HEXB gene codes for part of two essential nervous system enzymes: the beta subunit of hexosaminidase A and the beta subunit of hexosaminidase B. When there is a mutation in the coding for beta subunit of hexosaminidase A and the beta subunit of hexosaminidase B both enzymes do not function properly and lead to an accumulation of GM2 which is toxic and eventually causes cell death. Tay-Sachs is characterized by loss of function of only the alpha subunit of the hexosaminidase A enzyme. For more information: http://ghr.nlm.nih.gov/gene=hexb