Pluripotent embryonic stem (ES) cells are undifferentiated cell lines derived from early embryos and are capable of unlimited undifferentiated proliferation in vitro. They retain the ability to differentiate into all cell types including germ cells in chimeric animals in vivo, and can be induced to form derivatives of all three germ layers in vitro. Mouse ES cells represent one of the most important tools in genetic research. Major applications include the targeted mutation of specific genes by homologous recombination and the discovery of new genes by gene trap strategies. These applications would be of high interest for other model organisms and also for livestock species, However, in spite of tremendous research activities, no proven ES cells colonizing the germ line have been established for vertebrate species other than mouse a nd chicken thus far. This review summarizes the current status of deriving pluripotent embryonic stem cell lines from vertebrates and recent developments in nuclear transfer technology, which may provide an alternative tool for genetic modification of livestock animals. Copyright (C) 1999 S. Karger AG, Basel.