The transgenic construct comprised a pBR322-derived plasmid containing a 7.0-kb mouse genomic EcoRI fragment that included the mouse beta-major globin gene. In situ hybridization of a plasmid-derived probe to bone marrow metaphase chromosome spreads localized the integration site to the telomeric end of Chr 3. 800-1000 tandem copies of the transgene were estimated, by Southern blot analysis, to have integrated in this transgenic line. Northern blot analysis confirmed the expected lack (due to presence of the pBR322 sequences) of transcription of the exogenous globin genes, and analysis of blood from mice bearing the transgene detected no expression of the beta-major globin chain in their hemoglobin.