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A new murine homeo-box, called Hox-1.7, has been identified in a rare cDNA from F9 teratocarcinoma stem cells. The Hox-1.7 homeo-box is 68 and 72% homologous to the Drosophila antennapedia (Antp) and iab-7 homeo-boxes, respectively. A major 2.5-kilobase transcript and several minor transcripts were detected by Northern blot (RNA blot) analysis in adult tissues as well as in midgestational embryos. The posterior spinal cord was found to be a major site of Hox-1.7 expression in 12.5-day-old embryos. Somatic cell hybrids were used to map the Hox-1.7 gene to mouse chromosome 6. Restriction fragment length polymorphisms associated with either the Hox-1.7 gene or the previously known Hox-1 complex were identified. Their distribution patterns in recombinant inbred mouse strains were used to determine the linkage between the two loci as well as to other loci on chromosome 6. This maps Hox-1 and Hox-1.7 close to two mouse loci that affect morphogenesis, postaxial hemimelia (px) and hypodactyly (Hd).

Insertional mutagenesis was investigated in a transgenic mouse strain (HUGH/4) derived from a fertilized egg injected with plasmid DNA containing the human growth hormone gene. Lethality occurred in homozygous embryos and was traced to the egg cylinder stage on days 4 to 5 of gestation, shortly after implantation. The mutation is on chromosome 12 and is distinct in location and integration pattern from another mutation also leading to lethality of homozygotes in the egg cylinder stage. Based on this and other evidence, relatively many genes may be recruited to activity near the time of implantation and may therefore present a large target of vulnerability to mutagenesis. The single insert in HUGH/4, consisting of approximately three tandem copies of plasmid sequences, is flanked by mouse cellular sequences that have undergone rearrangements, including a probable deletion. The data suggest the hypothesis that DNA rearrangements, which appear to be commonplace in transgenic mice, may arise because the initial insertional complex is unstable; stepwise changes may then be generated until a more stable conformation is achieved.