Figure 1: Cross-inhibitory interaction between Pax4 and Arx promotes endocrine cell subtype allocation. Pancreatic progenitors expressing the transcription factor Pdx1 become fated to endocrine and exocrine compartments. The endocrine cell program is initiated by the activation of the b-HLH transcription factor Ngn3. Endocrine progenitors are allocated in a first round of cross-inhibitory interactions between Pax4 and Arx to a beta-/delta-cell fate or an alpha-cell destiny, respectively. Similarly, a second round of repressive interaction between Pax4 and an unknown factor X will promote the differentiation into beta- and delta-cells, respectively.