CXCR4 and lymphopenia: On the other hand, some of their symptoms were not detected in Wbp1l−/− mice, including blood neutropenia and lymphopenia and reduced spleen size.20 Increased CXCR4 expression and CXCR4‐mediated signalling were also observed in mice deficient in the expression of BAR domain containing adaptor protein Missing In Metastasis (MIM).