CXCL12 and 22q11.2 deletion syndrome: Following up on previous work, we established that Cxcl12 expression in pharyngeal surface ectoderm and rostral mesoderm is reduced in the absence of Tbx1. We then asked three main questions with regard to the potential role of the CXCL12:CXCR4 pathway in 22q11DS: to what extent do the defects in CXCL12 signalling mutants recapitulate Tbx1 mutation?