ROBO1 and breast cancer: Ubiquitin‐specific protease 33 (USP33), a member of ubiquitin‐specific protease family, was initially identified as a substrate molecule which binds to VHL E3 ligase.21 Previous studies showed that USP33 is a Robo1‐interacting protein that is involved in Slit signalling in midline axons crossing.22 Furthermore, USP33 is required for Slit‐Robo signalling in inhibiting breast cancer cell migration.15 Together, these studies demonstrate that USP33 plays an important role in the Slit‐Robo pathway.