Fig. 4. Pathways of islet-subtype specification. A hypothetical lineage diagram for Ngn3⁺ endocrine precursors, which give rise to all islet cell types (here, a single Ngn3⁺ cell is depicted giving rise to each subtype, whereas, in reality, the potential of a given Ngn3⁺ cell may be more restricted). In red are genes required for various steps of this process; some of which (top) appear to function similarly in all subtypes, constituting a core program of endocrine development, whereas others (bottom) are differentially required for specific subtypes (also see Table 1). Although more genes have been implicated in ß-cell development than in that of other subtypes, this is probably due to the greater effort focused on this cell type, rather than an inherently greater complexity in its developmental program.