Cotton fibers are single-celled structures that differentiate from epidermal cells of the outer integument of the seed coat that elongate, fill with secondary cell wall cellulose, mature and die. Studies, which include the analysis of gene expression throughout fiber development and the detection of fiber quality QTLs, are underway in many laboratories to identify loci arid/or genes of interest that might explain the underlying mechanisms that determine cotton fiber development and quality. The model plant Arabidopsis thaliana is being used to investigate the molecular basis of many important biological functions, some of which, such as cell differentiation, cell elongation, and cellulose biosynthesis and deposition are relevant to the development of cotton fiber cells and orthologous genes active in these processes might provide useful candidates for genes that determine fiber quality traits. Cellulose synthesis, in particular, could play an important role in determining certain aspects of cotton fiber quality, such as fiber length, strength, at- fineness. Recent advances in understanding the synthesis of cellulose hove come from the analysis of A. thaliana cellulose-deficient mutants, and the isolation of a number of genes involved in this process, including the 10 members of the catalytic sub-unit of the cellulose synthase (CESA). Database searching has identified a number of cotton EST CESA orthologs that cluster with A. thaliana sequences rather than together, allowing the identification of orthologs for five of the 10 A. thaliana CESA isoforms. Furthermore, one totton EST formed a branch on its own, possibly indicating a specialized function for this gene in fiber development. Other genes involved in cellulose biosynthesis, as well as genes involved in cell elongation or in maintaining polarized cell expansion have been identified in A. thaliana. and for most of these genes, cotton orthologs can be found among fiber ESTs. These orthologs represent useful candi