A proposed model of the regulation of lymphoid and myeloid lineage choice by MPPs. Schematic representation of GPCR-dependent regulation of lymphoid lineage specification at the VCAM-1⁻RAG1⁻ MPP population based on the results in this study. We propose that relocation of VCAM-1⁻RAG1⁻ MPPs marks the onset of lymphoid specification, and is necessary for proper lymphocyte development before the lymphoid lineage commitment stage.