CXCL12 and cancer: In a breast tumour xenograft model, resident human breast fibroblasts gradually transformed into CAFs under the stimulation of TGF‐β and stromal cell‐derived factor‐1 (SDF‐1), which initiate and maintain the conversion of fibroblasts into activated myofibroblasts (myCAFs) and cancer‐promoting phenotype during tumour progression.6