The endosperm is an essential part of the seed which sustains embryo development and contains storage reserves. Endosperm development involves a series of nuclear divisions in the absence of cytokinesis. In this study, it is shown that a mutant Arabidopsis plant carrying the disrupted At4g24900 gene exhibits severe defects during seed development. At4g24900 encodes a nuclear-localized C2H2-containing protein and the transcripts of this gene are alternatively spliced, consequently producing at least nine differentially spliced isoforms. The heterozygous T-DNA insertion mutation in this gene resulted in abortion of 25% of seeds, and the homozygous mutant allele displayed embryo lethality. Differential interfernce contrast (DIC) analyses revealed that the mutant has endosperm nuclei with more than one nucleolus and an embryo arrested at the globular to heart stage transition. Because this mutant exhibits a titan-like phenotype, it was designated ttl. The TTL gene is preferentially expressed in tissues with quickly dividing cells as revealed in P(TTL)::GUS (β-glucuronidase) transgenic plants. It is proposed that TTL is likely to function as a key regulator of endosperm nuclear division.