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There's many a CLP on the path to B.

Lymphocyte development is an excellent model system to study cell fate choices and the underlying transcriptional mechanisms. However, the earliest events have been poorly understood, due in part to the rarity of the relevant cellular intermediates. In this issue of Blood , Mansson and colleagues identify several subsets within the CLP and fraction A population (B220 + CLP phenotype) with differing lineage potentials. 1 The results indicate that B-lineage restriction initiates at much earlier developmental stages than previously thought and provide insight into the hierarchical transcriptional mechanisms involved in B-lineage commitment.