The low-affinity leukaemia inhibitory factor receptor (LIF-R) is believed to be required for the biological activities of a family of cytokines including LIF, ciliary neurotrophic factor (CNTF), oncostatin (OSM) and cardiotrophin-1 (CT-1). These are pleiotropic cytokines which have effects on a variety of cell types. Two important activities are as self-renewal factors for pluripotential embryonic stem (ES) cells and as survival factors of neutrons. To investigate requirements for LIF-R signalling in vivo, mice deficient for LIF-R have been generated via homologous recombination in ES cells. Homologous recombination in ES cells was achieved at high frequency (11/58) using a promoterless targeting construct. In order to ensure a null mutation, a 20Kb segment of the lif-r gene encoding the presumed ligand binding domain was deleted and replaced by an Internal Ribosome Entry Site-LacZ-neo-polyA (IRESβgeopA) cassette. This places the expression of βgeo under the control of LIF-R regulatory sequences. Therefore βgeo serves as a selectable marker and LIF-R expression reporter. Germline transmission has been obtained from two independent clones. Expression of the lacZ reporter gene integrated into the targeted locus has been visualized by histological staining. In vitro, β-galactosidase activity is predominantly confined to undifferentiated ES cells, although particular sub-populations of differentiated cells also stain. In vivo, staining is seen in specific regions of the developing nervous system, gut, heart, cartilage and muscles. Mice heterozygous for the mutated allele showed no overt phenotype. Homozygous newborns, however, were runted, lacked motility and died within the first postnatal day. Examination of the nervous system of LIF-R mutant mice showed specific β-galactosidase staining in motor neurons of the facial nucleus and spinal cord as well as neurons of the nucleus ambiguus. Neuronal counting of neonatal LIF-R (-/-) mice revealed a severe loss of neurons in these structures, which probably accounts for the observed lethality.