The maize ligule and auricle are structures on the maize leaf that develop at the boundary of the sheath and blade. In the absence of liguleless1 (Ig1) gene expression, ligule and auricle are not formed, and the blade-sheath boundary does not develop as an exact line between sheath and blade. By using the Activator (Ac) transposable element as a molecular tag, a novel Ig1 allele, Ig1-m1, was isolated and cloned. Analysis of somatic revertant sectors confirmed that the LG1 gene product functions in a cell-autonomous fashion. cDNA cloning as well as RT-PCR analysis of the LG1 mRNA indicate that the Ig1 gene is expressed at very low levels in the ligular region of developing maize leaf primordia, perhaps as early as plastochron 6 or earlier. Cellular localization studies in a heterologous system indicate that the LG1 product localizes exclusively to the nucleus. The predicted amino acid sequence of the LG1 protein is largely novel but contains an internal domain of 77 amino acids with significant similarity to a domain present in two recently identified SQUAMOSA PROMOTER-BINDING proteins 1 and 2 (SBP1 and SBP2) in Antirhinum majus.