Insertion of enhanced green fluorescent protein into the lysozyme gene creates mice with green fluorescent granulocytes and macrophages

Pluripotent hematopoietic stem cells have been studied extensively, but the events that occur during their differentiation remain largely uncharted. T o develop a system that allows the differentiation of cultured multipotent progenitors by time-lapse fluorescence microscopy, myelomonocytic cells wer e labeled with green fluorescent protein (GFP) in vivo. This was achieved b y knocking the enhanced GFP (EGFP) gene into the murine lysozyme M (lys) lo cus and using a targeting vector, which contains a neomycin resistant (neo) gene flanked by LoxP sites and "splinked" ends, to increase the frequency of homologous recombination, Analysis of the blood and bone marrow of the l ys-EGFP mice revealed that most myelomonocytic cells, especially mature neu trophil granulocytes, were fluorescence-positive, while cells from other li neages were not, Removal of the neo gene through breeding of the mice with the Cre-deleter strain led to an increased fluorescence intensity. Mice wit h an inactivation of both copies of the lys gene developed normally end wer e fertile.