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The vertebrate eye formation is a very complex phenomenon that relays on a sequence of inductive interactions. The eye development begins with the formation of the optic vesicle, a protrusion of the lateral forebrain. The optic vesicles upon contact with the over1ying ectoderm will determine the formation of the lens placode. At this stage the optic cup is formed from v-.t1ichthe optic stalk, pigmented epithelium and neural retina will arise. The chick embryo is an ideal model to study these ear1y steps of eye formation, since it is accessible to surgical manipulations and it allows rapid attempts of interference with the expression of specific genes. Transcription factors in general and homeobox genes in particular, are fundamental for the genetic control of different basic developmental processes (Gehring et ai., 1994). Therefore,the study of thesegenes might be crucial to understand the steps leading to the formation of a well organised and functional eye. Recently, several homeobox genes have been detected at ear1yand later stages of vertebrate eye and retinal development (Saha et ai, 1992; Halder et ai, 1995). Many of them A homologues of genes kno'M1 to be crucial in the development of the Drosophila compound eyes. Thus, in the search for new genes that might control the ear1y steps of vertebrate eye formation, we have looked for the chick . homologue of the Drosophila gene sine c-slx3 oculis. Sine oculis is a homeobox containing gene that, in the fly, appears to be fundamental for the development of the entire visual system (Cheyette et ai., 1994). Using t"-O different pairs of degenerate primers, we have amplified by nested PCR, the homeobox region of the homologue of sine oculis starting from a E2 chick cDNA GADPH library. Sequence analysis of the amplified fragments revealed the existence of at least four different genes related to the Drosophila sine oculis.The aa sequence of t"-O of them B were identical to the homeobox of the recently isolated murine six-1 and six-2 (Oliver et ai., 1995a). The remaining t"-O were used to screen the cDNA library and isolate t"-O cDNA inserts of 1.5 and 2.1 kb, respectively. Sequence analysis revealed that the first cDNA contained the entire coding sequence of a gene related to the Drosophila sine oculis, and highly homologue to that of the recently described mouse six-3 (Oliver et ai., 1995b). The second contained part of the coding sequence and the 3' end region of a gene relatedto the Drosophilasine oculis, but different from those so far described. We named them csix-3 and cSix-4, respectively. The csix-3 gene presented a homeodomain Identical to that of the mouse six-3 (70% homology with that of sine oculis). Et.5 E3 E T "P'!!'~__' "'.. E5 E T E7 E T E9 E T Ell

Cloning and characterisation of two chick homeobox genes, members of the six/sine oculis family, expressed during eye development.