• FMR1
• FMR2
• mosaicism
• fragile X syndrome

Fragile X syndrome is the most common disorder causing mental retardation with an estimated prevalence of between 1 in 4000 to 1 in 6000 males.^{1, 2} In nearly all cases, the molecular basis of this X linked mental impairment is an amplification in excess of 200 copies of an unstable CGG trinucleotide repeat sequence within the 5` untranslated region of the FMR1 gene (FRAXA, MIM 309550) at Xq27.3.³ This causes an inhibition of the transcription leading to a loss of function effect in the male. In the female, the amplified gene is preferentially X inactivated by methylation.⁴ However, the X inactivation pattern can vary between different cell types and during development and thus, usually, causes a milder phenotype in female patients.⁵ In close distal proximity to the FMR1 gene, a second gene, FMR2 (FRAXE, MIM 309548) involved in mental retardation, has been described with amplification of an unstable GCC trinucleotide repeat sequence as the underlying pathological mechanism.⁶ This report adds a further case with mosaicism for a FMR1 and FMR2 deletion to the few described cases with a large FMR1 deletion.^7–11