The Meaning of CAG

All persons have two HD genes, one from mom and one from dad. The genetic measure of a bad gene for HD is a repeated genetic sequence, C-A-G-C-A-G-C-A-G ..., repeated 36 or more times. If mom or dad has HD then there is a 50-50 chance (like the flip of a coin) that each kid will inherit the bad gene. The number of repeats of the C-A-G sequence is the CAG count found by the genetic test.

CAG Count and The Age of Onset of HD
For a Phd (Person with HD) with a CAG count of 36 or more there is a wide variation in the age of onset.

CAG Count and The Duration of HD
There is no relationship between CAG count and the duration of HD.