Fig. 5. Two models for the concerted combinatorial action of Vax2 and other transcriptional regulators along the nasal-temporal (NT) axis of the ventral retina. (A) Ventral regulators are normally expressed at higher levels in the nasal than the temporal ventral retina, and a minimum aggregate level of the regulators (indicated by the striped bar) is necessary for proper retinocollicular mapping. When one of the regulators (e.g. Vax2) is removed by mutation, the aggregate level becomes limiting (falls below the threshold bar) only in the ventral temporal retina. (B) Vax2 (left discs), which is normally steeply graded along the DV axis, genetically interacts with one or more genes that are graded along the NT axis (middle discs), to form a composite wild-type map of axon guidance regulators (right discs). With the loss of Vax2, the map reverts to that of the NT genes alone, which again most severely affects ventral temporal RGCs. See text for details.