Eotaxin is required for the differentiation of mast cell progenitors expressing mMCP-4. RT-PCR was used to determine the pattern of mast cell proteases (carboxypeptidase A, mMC-CPA; cellular protease-2, mMCP-2; and cellular protease 4, mMCP-4) expressed in FBL colonies after exposure to the factors indicated in the figure (see Materials and Methods for details). Lanes 1 and 2 represent BMCMC derived from BALB/c mice, in SCF alone (lane 1) or Con-A–stimulated conditioned medium as a source of IL-3 (CM; lane 2). Lanes 3 to 6 represent FBL-derived colonies (11 dpc).