Fig. 8. Models describing the two possible modes of regulation of expression of EphB2, EphB3 and ephrin B2 by RA, Vax and Tbx5. Model 1: Vax and Tbx5 function in the dorsal and ventral retina to regulate the expression of ephrin B2, EphB2 and EphB3 exclusively through the RA synthesizing enzymes, RALDH1 and RALDH3. Model 2: Vax and Tbx5 function independently of RA to regulate expression of EphB2, EphB3 and ephrin B2. According to this model, both Tbx5-dependent and RA-dependent positive inputs are required to express ephrin B2 in the dorsal retina. The enhancer regions of EphB2 and EphB3 would require positive inputs from both Vax and RA for their expression in the ventral retina.