Editor—Retinitis pigmentosa (RP, MIM 268000) is the term applied to a clinically and genetically heterogeneous group of retinal degenerations primarily affecting the rod photoreceptors. RP is characterised by progressive loss of vision, initially manifesting as night blindness and reduction in the peripheral visual field, and later involving loss of central vision.1 Ophthalmoscopic examination typically shows pigmentary disturbances of the mid-peripheral retina. RP may be inherited as an autosomal recessive, autosomal dominant, digenic, or X linked trait. Autosomal recessive RP (ARRP) accounts for around 20% of all cases of RP, while sporadic RP, which is presumed to be recessive in most cases, accounts for a further 50%.2

Mutations causing autosomal recessive RP (ARRP) have been found in the genes encoding rhodopsin,3 the α and β subunits of rod phosphodiesterase,4 5 the α subunit of the cyclic GMP gated channel protein,6 and the genesRPE65,7 8 RLBP1,9 ABCR,10 andTULP1.11 12 In addition, genetic linkage studies have identified ARRP loci at 1q31-q32.1,13 14 2q31-q33,156cen-q15,16 and 16p12.1-p12.3.17 (MIM numbers for the loci identified by these studies are 180380, 180071, 180072, 123825, 180069, 180090, 601708, 600132, 600105, and 602594 respectively. No MIM number has yet been assigned to the 2q31-q33 and 6cen-q15 loci.) Of these four loci, only linkage to the 1q31-q32.1 locus (RP12) and the 6cen-q15 locus has been reported in more than one pedigree.

The first report of linkage of ARRP to 1q31-q32.1 was in a large inbred Dutch family with ARRP in which most patients exhibited para-arteriolar preservation of the retinal pigment epithelium (PPRPE).13This was followed by linkage of a second, consanguineous, pedigree from Pakistan.14 In both the Dutch and the Pakistani families there was evidence that only …