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Equine herpesvirus 1 (EHV-1) is a major cause of respiratory disease and abortion in horses and is associated with rarer conditions including a neurological syndrome. Restriction endonuclease analyses indicated the existence of two virus subtypes (Sabine et al., 1981 ;Studdert et al., 1981), which supports the results of earlier studies on the serology and pathogenesis of different virus isolates. Cullinane et al. (1988) demonstrated that the two subtype genomes are collinear and have the same structure : a long unique sequence (U L ) linked to a short unique sequence (U S ) flanked by an inverted repeat (TR S \IR S ). Summation of the component sizes (109 kbp for U L , 13n4 kbp for U S and 10n8 kbp each for TR S and IR S ) yielded an estimate for the subtype 2 genome size of 144 kbp, which is somewhat smaller than that for subtype 1 (150 kbp ; Whalley et al., 1981). Cullinane et al. (1988) also showed that restriction maps for subtype 2 differ from those for subtype 1. The data were thus sufficient to support classification of the two

Approximately one in 2000 children is born with a genetic hearing impairment, mostly inherited as a non-syndromic, autosomal recessive trait, for which more than 30 different genes have been identified. Previous studies have shown that one of these genes, connexin 26 (GJB2), accounts for 30-60% of such deafness, but the relative contribution of the many other genes is not known, especially in the outbred UK population. This lack of knowledge hampers the development of diagnostic genetic services for deafness. In an effort to determine the molecular aetiology of deafness in the population, 142 sib pairs with early-onset, non-syndromic hearing impairment were recruited. Those in whom deafness could not be attributed to GJB2 mutations were investigated further for other mapped genes. The genetic basis of 55 cases (38.7%) was established, 33.1% being due to mutations in the GJB2 gene and 3.5% due to mutations in SLC26A4. None of the remaining 26 loci investigated made a significant contribution to deafness in a Caucasian population. We suggest that screening the GJB2 and SLC26A4 genes should form the basis of any genetic testing programme for childhood deafness and highlight a number of important issues for consideration and future work.