For instance, it is possible that when two TFs are expressed together in the same single cell they lead to a certain cellular outcome. However, these same two factors might also be expressed separately in other single cells. A combination of more sophisticated algorithms and functional studies will be necessary to fully understand the extensive heterogeneity of TF expression in developing RPCs. Neurogenic basic helix-loop-helix transcription factors have been shown to play crucial roles in the generation of many postmitotic retinal cell types. Recently the loss of one bHLH, Math5,Oleuropein was shown to lead to deficiencies in cell cycle progression in RPCs, revealing a possible additional coordinating role for this class of TF in RPCs. Understanding the mechanism of action of these bHLH factors requires a detailed knowledge of their expression patterns. In the 42 single RPC profiles, the neurogenic bHLH genes were found in subsets of cells. For example, at least three cells at three different timepoints expressed significant levels. However, these previous conclusions were based upon upregulation of these bHLHs in the absence of Math5 and not a direct observation of their co-expression. Interestingly,Imperialine-D-glucoside single cell RTPCR in the chick retina revealed that a few cells could co-express certain bHLHs. It is possible that these reporters did not fully recapitulate the entire spectrum of expression for these genes, perhaps due to differences in the regulation of transcription or translation, as has been shown for certain homeobox TFs in Xenopus. These single cell profiles demonstrate the expression of multiple neurogenic bHLHs in single RPCs and suggest that the interplay among these TFs is perhaps not as simple as previously postulated. These data provide a potential explanation for the observed redundancy of these bHLH factors in retinal development. Furthermore, Xenopus NeuroD1 has been shown to be regulated by phosphorylation and if similar regulatory mechanisms exist in the mouse, this could provide a method for independently controlling bHLHs that are coexpressed. Additionally, these TFs are part of much larger families of factors and many other family members were found in subsets of RPCs as well.