Coordinated cell growth and differentiation is crucial for the development of higher plants. Using the elongation defective 1-1 (eld1-1) mutant, we cloned the ELD1 gene, which encodes a serine-rich protein. Genes homologous to ELD1 can be found in plants, including Arabidopsis, rice, and tobacco, but not in other organisms. Using reverse genetics, we identified a new allele, eld1-2, which is phenotypically indistinguishable from eld1-1, but does not produce a detectable ELD1 transcript. The ELD1 gene sequence is the same as that of the KOBITO1 sequence. However, the kob1 mutants display weak phenotype relative to the two eld1 mutants, which are likely null alleles. KOB1 was reported to be a membrane protein involved in cellulose synthesis. However, based on ELD1-GFP localization in plasmolyzed cells, we found that ELD1 is localized to the cell wall/extracellular matrix, rather than the membrane. Thus, ELD1/KOB1 is a secreted protein involved in promoting cell growth. To investigate the relationship between ELD1 and Arabidopsis genes with high sequence similarity, we analyzed the possible subcellular location of their proteins as well as their amino acid sequence. The ELD1-related proteins in Arabidopsis were predicted to be localized to subcellular compartments different from that of ELD1. Thus, ELD1 is likely to be functionally distinct from related Arabidopsis genes. These results suggest that ELD1 is a single-copy gene which belongs to a small family of plant-specific genes with diverse function.