Knocking Out Complement Genes
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The modification of chromosomal loci by homologous recombination is commonly referred to as gene targeting, the importance of which needs no emphasis. Gene targeting has been widely used in mice to make a variety of mutations in many different loci, including those of the complement components, so that the phenotype of genetic modifications may be assessed in the entire organism. Gene targeting was first achieved in mammalian cells in 1985 (1 ,2 ). With the isolation of embryonic stem (ES) cells from the inner cell mass of early mouse embryos (3 ,4 ) and from the realization that these cells could then be cultured for a substantial period of time and still contribute to the germline of a mouse after reintroduction into a blastocyst, even after genetic modification (5 ,6 ), a powerful new method for analyzing gene function in vivo has emerged.
