1.5 Mutational spectrum Choroideremia is an X-linked recessive inherited chorioretinal dystrophy caused by mutations in the CHM gene, this spans a genomic sequence of B150 kb on chromosome Xq 21.2, contains 15 exons and encodes a ubiquitously expressed protein of 653 amino acids; Rab Escort Protein 1 (REP1). REP1 is an essential component of the catalytic Rab geranyl-geranyl transferase (RGGTase) II complex, and is involved in the regulation of intracellular membrane transport traffic. There have been over 130 unique mutations in CHM reported to date (web-based database http://www. lovd. nl/CHM). Characterisation of the mutation spectrum reveals deletions, insertions, duplications, translocations, nonsense, splice-site, frameshift and missense mutations. Full gene and partial deletions represent 25–50% of mutations, and a further 30% are nonsense mutations resulting in premature termination codons. Deletions vary in size from a few kilobases removing a single exon to B15Mb comprising the entire CHM gene and large parts of chromosome Xq21. 1 Two missense mutations have been reported: using in silico analysis, the c. 1679 T> C (p. L550P) mutation was predicted to destabilise the β-structural elements and tertiary structure resulting in absence of REP1 in patient lymphocytes; 2 the c. 1520A> G (p. H507R) missense was found to generate a functionally inactive REP1 variant that was not capable of interacting with RGGTase. 3 Carrier females are generally asymptomatic but funduscopic examination often shows patchy areas of chorioretinal atrophy that represent clonal areas of the disease due to random X-inactivation. However, later in life, carrier females can often develop night blindness and field loss because of expanding areas of chorioretinal atrophy. Translocations between the X-chromosome and an autosome, disrupting CHM have been detected in females (but not males), displaying mild clinical signs of choroideremia and ovarian dysgenesis; t (X; 7)(q21. 2; p12), t (X; 13)(q21. 2; p12) and t (X; 4)(q21. 2; p16. 3). 4–6 A fourth complex translocation involving chromosomes X, 1 and 3: t (X; 1; 3)(q13; q24; q21), inv (9)(p11q13) has been identified in a female carrier with a severe choroideremia phenotype (thought to be due to nonrandom inactivation of the normal X-chromosome) and ectodermal dysplasia, with no comment of gonadal dysgenesis. 7

1.6 Analytical methods Bi-directional fluorescent Sanger sequencing of coding and intron–exon boundaries of CHM is the mainstay analytical method. 8 However, a multiplex ligation-dependent probe amplification assay (MLPA) has been developed to test for deletions and duplications within the CHM gene, and this is particularly useful in suspected heterozygotes. 9 Sequence variants are described following HGVS nomenclature guidelines (http://www. hgvs. org/) relative to the NCBI reference sequence NM_000390. 2.