MUSK and breast cancer: Indeed, this link is supported by the fact that six out of eight RTKs upregulated in lapatinib-resistant mammary tumors—Axl, VEGFR1, MUSK, PDGFRβ, PDGFRα, EPHA2—are known Hsp90 clients (www.picard.ch/downloads/Hsp90interactors.pdf).