Chromosome 19 has the highest gene density of all human chromosomes, more than double the genome-wide average. The large clustered gene families, corresponding high G1C content, CpG islands and density of repetitive DNA indicate a chromosome rich in biological and evolutionary significance. Here we describe 55.8 million base pairs of highly accurate finished sequence representing 99.9% of the euchromatin portion of the chromosome. Manual curation of gene loci reveals 1,461 protein-coding genes and 321 pseudogenes. Among these are genes directly implicated in mendelian disorders, including familial hypercholesterolaemia and insulin-resistant diabetes. Nearly one-quarter of these genes belong to tandemly arranged families, encompassing more than 25% of the chromosome. Comparative analyses show a fascinating picture of conservation and divergence, revealing large blocks of gene orthology with rodents, scattered regions with more recent gene family expansions and deletions, and segments of coding and non-coding conservation with the distant fish species Takifugu.

We describe the cloning and function of the human XRCCI gene, which is the first mammalian gene isolated that affects cellular sensitivity to ionizing radiation. The CHO mutant EM9 has 10-fold-higher sensitivity to ethyl methanesulfonate, 1.8-fold-higher sensitivity to ionizing radiation, a reduced capacity to rejoin singlestrand DNA breaks, and a 10-fold-elevated level of sister chromatid exchange compared with the CHO parental cells. The complementing human gene was cloned from a cosmid library of a tertiary transformant. Two cosmid clones produced transformants that showed -100% correction of the repair defect in EM9 cells, as determined by the kinetics of strand break repair, cell survival, and the level of sister chromatid exchange. A nearly full-length clone obtained from the pcD2 human cDNA expression library gave =80% correction of EM9, as determined by the level of sister chromatid exchange. Based on an analysis of the nucleotide sequence of the cDNA insert compared with that of the 5' end of the gene from a cosmid clone, the cDNA clone appeared to be missing -100 bp of transcribed sequence, including 26 nucleotides of coding sequence. The cDNA probe detected a single transcript of -2.2 kb in HeLa polyadenylated RNA by Northern (RNA) blot hybridization. From the open reading frame and the positions of likely start sites for transcription and translation, the size of the putative XRCC1 protein is 633 amino acids (69.5 kDa). The size of the XRCCI gene is 33 kb, as determined by localizing the endpoints on a restriction endonuclease site map of one cosmid clone. The deduced amino acid sequence did not show significant homology with any protein in the protein sequence data bases examined.The processes that repair DNA damage, as well as those responsible for the rearrangement of chromosomal DNA through recombinational events, are of fundamental importance in mutagenesis and carcinogenesis. In Escherichia coli, a large number of genes have been identified that participate in repair and recombination (15,55,56). In the simpler eucaryotes Saccharomyces cerevisiae and Drosophila melanogaster, the genetic analysis of these systems is less advanced, but here too progress has been made in identifying many genes that participate in repair processes (5,15,34). Progress has been much slower with mammalian cells because of the larger sizes of genes, longer cell doubling times, and intrinsic limitations on performing genetic analyses.The use of repair-deficient rodent cell mutants has proved to be the most successful approach to the molecular cloning of human DNA repair genes (45). Mutants of CHO cells that are defective in the nucleotide excision repair pathway (48, 61) have provided a tool for isolating the human genes ERCCI (53, 60), ERCC2 (57, 58), ERCC3 (59), ERCC5 (29), and ERCC6 (18) on the basis of functional complementation of the mutations by transfection of human genomic DNA, followed by cosmid or bacteriophage vector cloning. These five ERCC (excision repair cross-complementing) genes correct mutations that ...

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