We report the construction of the first complete genetic linkage map of the laboratory rat. By testing 1171 simple sequence length polymorphisms (SSLPs), we have identified 432 markers that show polymorphisms between the SHR and BN rat strains and mapped them in a single (SHR x BN) F2 intercross. The loci define 21 large linkage groups corresponding to the 21 rat chromosomes, together with a pair of nearby markers on chromosome 9 that are not linked to the rest of the map. Because 99.5% of the markers fall into one of the 21 large linkage groups, the maps appear to cover the vast majority of the rat genome. The availability of the map should facilitate whole genome scans for genes underlying qualitative and quantitative traits relevant to mammalian physiology and pathobiology.

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The targeted inactivation of oncogenes may be a specific and effective treatment for cancer. However, because human cancers are the consequence of multiple genetic changes, the inactivation of one oncogene may not be sufficient to cause sustained tumor regression. Moreover, cancers are genomically unstable and may readily compensate for the inactivation of a single oncogene. Here we confirm by spectral karyotypic analysis that MYC-induced hematopoietic tumors are highly genetically complex and genomically unstable. Nevertheless, the inactivation of MYC alone was found to be sufficient to induce sustained tumor regression. After prolonged MYC inactivation, some tumors exhibited a distinct propensity to relapse. When tumors relapsed, they no longer required the overexpression of MYC but instead acquired novel chromosomal translocations. We conclude that even highly genetically complex cancers are reversible on the inactivation of MYC, unless they acquire novel genetic alterations that can sustain a neoplastic phenotype. (Blood.

The hepatocyte growth factor (HGF) regulates biological functions such as cell growth, cell migration, and morphogenesis by activation of its tyrosine kinase receptor Met.1 The HGF receptor/Met is a 190-kd heterodimeric transmembrane receptor composed of a 50-kd ␣-and a 140-kd ␤-subunit. The ␤-subunit is a transmembrane spanning protein with an intracellular tyrosine kinase domain that can be activated by autophosphorylation of two tyrosine residues.2 Although the Hgfr/Met proto-oncogene was originally identified as a transforming gene isolated from a carcinogen-treated human osteosarcoma cell line, 3 it is primarily expressed in epithelial cells. Many types of human carcinomas overexpress this receptor, which suggests this receptor is involved in the development and progression of epithelial tumors. 4 -6 Because HGF is ubiquitously expressed in mesenchymal tissues and released to the extracellular compartment, paracrine and/or autocrine signaling would be a possible mechanism of tumorigenesis in mesenchymal tumors that express the HGF receptor. 7,8 Recent studies have shown that the Hgfr/Met proto-oncogene is fre-

The chromosome banding pattern was analyzed in bone-marrow cells and/or spleen cells of 10 patients in the blastic phase of chronic myeloid leukemia (CML). It was obvious from the karyotype analysis that the chromosome aberrations occurring addition to the Philadelphia chromosome (Ph1) were strictly non-random. An extra Ph1, trisomy 8 and/or trisomy for the long arm of chromosome 17 were observed in all cases. This consistent pattern of chromosome involvement in CML was confirmed in 57 cases from the literature studied with banding techniques. In 88% of the total number of cases with further changes at least one of the three main chromosomal aberrations was found ("major route" of karyotypic evolution).

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A rat line was generated in which genomic integration of a ROSA-EGFP transgene resulted in exclusive expression of EGFP in the germ cells of both sexes. EGFP expression was uniform and robust in cleavage stage embryos beginning at the late 2-cell stage and continuing through blastocyst development where expression became restricted to cells of the inner cell mass. Subsequent analysis showed high EGFP expression exclusively in primordial, embryonic, and adult germ cells. This unique expression pattern makes this EGFP marked locus the first molecular marker of the germline lineage in both sexes in mammals. FISH was used to localize the transgene insertion to chromosome 11q11-q12, proximal to Grik1 and near Ncam2. Analysis of the region did not identify known germ cell-specific genes but did identify 19 ESTs or transcribed loci present in testes, ovary, or pre-implantation libraries from mice or rats. To assess the utility of the transgenic line for germ cell transplantation studies, non-selected, freshly isolated seminiferous tubule cells were transferred to the testis of recipient males. The donor cell population colonized the testis at a surprisingly high efficiency within 30 days following transfer. Since EGFP is a vital marker, the colonization process can be followed in vivo and the extent of colonization quantified. The unique germ cell specific expression of EGFP makes this line of transgenic rats an excellent novel tool to study germ cell origin, development, and differentiation, and to assess the plasticity of adult somatic stem cells to become male germ cells.