Cotton fibres are single epidermal cells developed from the ovule. While a number of genes expressed exclusively in cotton fibre development have been isolated, the genes responsible for differentiation of fibre cells have not yet been identified (Basra, 1999). Arabidopsis leaf trichomes could potentially serve as a model for investigating the genetic mechanism controlling cotton fibre differentiation, as the two developmental processes have several features in common. Arabidopsis trichome development is well characterised both genetically and molecularly, and provides an excellent model for plant cell differentiation for several reasons (Marks et al. , 1991). These reasons include ease of observation, relative simplicity of the developmental process, and viabillty of plants with mutant trichomes. One gene shown to be required for Arabidopsis trichome initiation is TRANSPARENT TESTA GLABRAl (TTGl) (Koornneef, 1981). This gene has recently been cloned and shown to encode a 341 amino acid protein with four WD40 repeats (Walker et al. , 1999). The TTGl locus regulates several pathways in Arabidopsis (in addition to leaf trichome formation), including anthocyanin pigmentation, root hair formation, and seed coat mucilage production. Due to the role played byTTGI Arabidopsis trichome initiation, it was thought homologues of TTGl might exist in cotton to control fibre initiation. Four putative TTGl homologues have been isolated from cotton, designated TTG(1),TTG(11), TTGOPl and pTTG(c)E2 (Orford, unpublished).