SPP1 and cancer: We further found that it was the V3–V7 exon region of CD44v to interact with OPN for cancer cell invasion, in that when the CD44v3–10 and CD44v8–10 isoforms were overexpressed individually in MCF10CA1h, only CD44v3–10, but not CD44v8–10, responded to OPN stimulation and promoted cancer cell invasion (Supplementary Figure 6B).