SORL1 and breast cancer: MDK is a heparin‐binding growth factor overexpressed in tissues of BC brain metastases (Figure S14A), CSF‐CTCs (Figure S14C) and BC tissues (Figure S14D,E), whereas there was no obvious difference of MDK expression in the five subtype‐specific BC tissues (Figure S14B), we have discovered MDK‐SORL1 and MDK‐LRP1 ligand–receptor pairs were significantly interacted in the five subtype‐specific CSF‐CTCs and macrophages (monocytes) (Figures 4C and S11).