Table 1.

Allele category definitions in common usage.

Allele	UK		USA
	Repeat size	Methylation status	Repeat size	Methylation status
Normal	0–45	Regular	0–44	Regular
Intermediate	46–58	Regular	45–54	Regular
Premutation	59–200 *	Regular	55–200 *	Regular
Full mutation	>200 *	Hypermethylated *	>200 *	Hypermethylated *

* Current techniques cannot give a precise sizing at the level of hundreds of repeats and there is an overlap of repeat sizes which may be methylated or unmethylated, so the distinction between premutation and full mutation is usually made by combining observations of size, methylation status and degree of somatic mosaicism; hence, there are examples of unmethylated full mutations as well as of premutations estimated at >200 repeats. “Unmethylated” is hitherto used to refer to the regular state of DNA in which CpG motifs are not methylated and therefore digestible by methylation-sensitive restriction enzymes.