Embryonic stem cell technology is now well established in the mouse (reviewed by Robertson, 1987). This technology implies the isolation from the preimplantation embrao of a cell line (ES) that is cultured in vitro in an undifferentiated state. Embryonal carcinoma cells (EC) lines obtained from malignant tumours (Martin, 1975), together with all the information available on their culture requirements (reviewed by Heath, 1987), represented a very important starting point for the establishment of ES cells (Martin, 1981). ES cells share many characteristics with EC cells such as the ability to contribute to somatic tissues of animals obtained following injection of cells into a host blastocyst, to differentiate in vitro under appropriate stimuli (Rudnicki & McBurney, 1987) and to form retransplantable tumours. ES cells, however, have substantial advantages over EC cells in that they can be derived directly from a normal embryo, they maintain a normal karyotype and when reintroduced into a host blastocyst they can colonise the germ line (Bradley, 1987). ES cells are maintained in an undifferentiated state by the presence of feeder layers producing various factor(s) that prevent to the cells from differentiating. It has been shown that glycoproteins are responsible for this effect and these have been named according to their different activities: DIA, differentiation inhibitory activity (Smith & Hooper, 1987); LIF, leukaemia inhibiting factor (Smith et al, 1988; Williams et al, 1988); HILDA, human interleukin for DA cells (Moreau et al., 1988). It is now possible to establish and maintain ES cells in culture in the absence of feeders cells but in the presence of such factors (Nichols et al., 1990).