International system for cytogenetic nomenclature of the domestic horse. Report of the Third International Committee for the Standardization of the domestic horse karyotype, Davis, CA, USA, 1996.

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The history of domestic horse (Equus caballus) cytogenetics dates from 1912 when Kirillow reported the diploid chromosome number to be between 20 and 34 in male horses (see Power 1990). Later, Krallinger (1931) and Makino (1942) reported the number to be 64. Although this provided the correct chromosome number, the prevailing uncertainty was ®nally cleared in 1959 by Rothfels and co-workers who con®rmed the correct chormosome number as 2n ˆ 64 (Rothfels et al. 1959). The horse has 13 pairs of metacentric/submetacentric and 18 pairs of acrocentric autosomes. The X chromosome is the second largest submetacentric, while the Y chromosome is one of the smallest acrocentrics. During the 1970s, G-, Q-, C-, Rand NOR banding techniques were applied to horse chromosomes, and several arrangements of the chromosomes into a karyotype were presented (see Power 1990). The First International Conference for the Standardization of Banded Karyotypes of Domestic Animals held at Reading, UK, 1976, proposed a G-banded horse karyotype (the Reading Standard) with eight rows of chromosomes (Ford et al. 1980). Biarmed autosomes were arranged by decreasing length in the ®rst four rows. Acrocentrics were similarly arranged in the next three rows. The last row had only the sex chromosomes. This standard karyotype was used for several years. The lack of complete consensus among horse cytogeneticists led to a new standard (the Paris Standard) de®ned in 1989 (Richer et al. 1990). The chromosomes were arranged into two groups ± nonacrocentrics and acrocentrics. Within each group the chromosomes were ordered by length. The sex chromosomes were placed in the middle of the karyotype, along with the smallest biarmed chromosomes. C-, Rand NOR banded karyotypes were also presented. The Paris standard karyotype was more compact, with relatively better band resolution than the Reading Standard. Despite acceptance of the Paris Standard, the quality of the published banded images was a matter of concern. Furthermore, idiograms, band numbers and landmark descriptions were not provided, limiting the use of the standard to chromosome identi®cation only. The need for schematic drawings and band designations became particularly obvious in 1988 with the ®rst reports of physical gene mapping in horses (MHC: Ansari et al. 1988, MaEkinen et al. 1989; CRC: Harbitz et al. 1990; GPI: Chowdhary et al. 1992; PGD: Gu et al. 1992). Even though G-band idiograms and band descriptions (Maciulus et al. 1984) were available before these mapping efforts, dif®culties were encountered in the precise de®nition of the band locations primarily because of the lack of consensus on landmarks and band numbers. Development of the physical and genetic Chromosome Research 1997, 5, 433±443