Mammalian chimaeras have proved useful for investigating early steps in embryonic development. However, a complete clonal analysis of cell lineages has been limited by the lack of a marker which is ubiquitous and can distinguish parental cell types in situ. We have developed a cell marker system which fulfils these criteria. Chimaeric mice were successfully producedfrom two mouse species which possess sufficient genetic differences to allow unequivocal identification of parental cell types. DNA-DNA in situ hybridization with cloned, species-specific sequences was performed to distinguish the parental cell types. We have identified a cloned, Mus musculus satellite DNA sequence which shows hybridization differencesbetween Mus musculus and Mus caroli DNA. This clone was used asa probe in in situ hybridizations to bone marrow chromosomes from Mus musculus, Mus caroli, and an interspecific F1 hybrid. The clone could qualitatively distinguish Mus musculus from Mus caroli chromosomes after in situ hybridization, even when they were derived from the same F1 hybrid cell. Quantitation of this hybridization to interphase nuclei from bone marrow spreads indicates that the probe can successfully distinguish Mus musculus from Mus caroli cells and can determine the percentage contribution of Mus musculus in mixtures of bone marrow cells of these species and in chimaeric bone marrow cell preparations.

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