Differentiation of glia (astrocytes and oligodendrocytes) in Drosophila requires the gene glial cells missing (gcm), which controls lineage determination. In the absence of gcm, neuroglia progenitors exclusively differentiate into neurons, instead of into both neurons and glia. In contrast, ectopic overexpression of gcm causes uniform differentiation of the neuroglia progenitors into glia. Glial and neuronal cells in vertebrates similarly derive from neuroblast progenitors. To investigate vertebrate glial formation, we have identified, cloned, and chromosomally mapped a mammalian gcm homolog. Mouse Gcm1 demonstrates extensive similarity to Drosophila gcm but is expressed at very low levels during neuro- and gliogenesis.