Rap1 has multiple functions in hematopoietic cells. Rap1 can enhance integrin-mediated adhesion in all hematopoietic cells examined to date. Rap1 can also regulate ERK activation in the 3 types of cells shown here (megakaryocytes, myelocytes, and T lymphocytes). Rap1 activates ERK via B-Raf in megakaryocytes and myelocytes that express B-Raf. In T cells that do not express B-Raf, Rap1 antagonizes Ras-dependent ERK activation. The proposed actions of Rap1 on ERK and adhesion and the overall cellular responses to Rap1 are listed below each cell type.