Previous analyses of tso1 mutants revealed a loss of control of directional cellular expansion and coordination of growth of adjacent cells, and defects in karyokinesis and cytokinesis. We isolated TSO1 using a map-based approach, and show that it is a member of a family of at least three genes in Arabidopsis. Consistent with the mutant phenotype, TSO1 transcript was most abundant in flowers, where it accumulated to the highest levels in developing ovules and microspores. The putative TSO1 protein has two cysteine-rich regions that are similar to the CXC domains of a variety of proteins from plants and animals, including a class of kinesins involved in chromosome segregation, and enhancer of zeste-type proteins. Visualization of TSO1-fusion proteins indicated that TSO1 is a nuclear protein. The tso1 mutant phenotypes and the novelty of the TSO1 sequence suggest the existence of previously unknown participants in regulation of directional processes in eukaryotic cells.