Alternatively, the mutual lineage switch could be caused by a more general mechanism. Perhaps when cells are becoming specified to one particular lineage a process becomes activated that leads to transcriptional silencing of many other lineage programs. For example, lineage-determining factors may have to compete for a finite amount of certain ubiquitously expressed and required co-factors, which would lead to suppression of undesired lineages once differentiating cells have committed to one lineage. E-proteins could potentially be one such critical co-factor as they are known to heterodimerize with many different lineage-specific bHLH transcription factors (Massari and Murre, 2000).
