[10] The most common mutation, which occurs in over 80 percent of Tay–Sachs patients, results from a four base pair addition (TATC) in exon 11 of the Hex A gene.

Defects in the HEXA gene, however, prevent this degradation, leading to a buildup of toxins in brain and spinal cord cells.

Because the Tay-Sachs gene defect mainly affects neural cells, a patient with the HEXA mutation will experience a quick deterioration of motor and mental function before dying around the age of three or four.

[citation needed] A “knockout” model, which is a mouse that has been genetically modified to observe the effects of inactivation of or damage to certain genes, found that the mice that were administered the HEXA gene experienced many of the same symptoms of Tay-Sachs, with one exception: GM2 buildup was distributed differently in the brains of the mice than in those of a typical human Tay-Sachs patient.

One study, done on mice, successfully reestablished beta-hexoaminidase levels and removed the toxic cell buildup by using a non-replicated Herpes simplex vector to code for the missing gene.