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Cover Chemokine receptor 4 (CXCR4) and Gab1, a docking protein involved in tyrosine kinase signaling, regulate progenitor cell migration during muscle development. Shown here is an immunofluoresence analysis of a wild-type mouse tongue section at embryonic day 13.5, using antibodies directed against myosin (green) and myoD (red). In CXCR4/Gab1 double-mutant mice, long-range migrating muscle progenitor cells do not reach the anlage of the tongue. As a result, the tongue muscle groups of somitic origin are lacking in such mutants. (For details, see Vasyutina et al., p. 2187.)