Table 1. Four classes of CGG repeat alleles.

Number of repeats	Allele range	Phenotype	Stability
<45 repeats The most common alleles contain 29 or 30 repeats	Normal (N)	Normal	Transition to a full mutated allele has never been reported. Extremely rare cases of minor changes in repeat number have been described
45–54 repeats	Intermediate or grey-zone allele (IA)	Normal	Possible instability upon transmission. Very rare cases of expansion to a premutation have been described. Very rare cases of expansion to a full mutation have been described in two generations but not in one generation
55 to ∼200 repeats without abnormal methylation	Premutation (P)	Risk of FXPOI for females. Risk of FXTAS for males and females	Unstable upon transmission and at risk to pass on a full mutation in one generation when transmitted by a female. This risk is proportional to the premutation size
>∼200 repeats with abnormal methylation	Full mutation (M)	Males are affected with FXS. ∼50% of females are affected with FXS

Repeat range for normal, intermediate, premutation and full mutation alleles and stability. Potential inaccuracy in sizing should be born in mind when interpreting the results of repeat sizing. It is reasonable to accept a maximum error of ±5% of the total repeat size, eg, 50±2, 60±3, 80±4, 100±5; the laboratory should establish the accuracy of its test. All sized quoted should be qualified as ‘approximately' in the report, unless the number of repeats has been determined directly (eg, by sequence analysis or comparison with a sequenced control).