Lim1 is a homeobox gene expressed in the organizer region of mouse embryos. To investigate the role of Lim1 during embryogenesis, a targeted deletion of the Lim1 gene was generated in embryonic stem cells. Embryos homozygous for the null allele lacked anterior head structures but the remaining body axis developed normally. A partial secondary axis developed anteriorly in some mutant embryos. Lim1 is thus an essential regulator of the vertebrate head organizer.