Class 3 semaphorins (A-G) bind NRPs. They repel axons and collapse growth cones. SEMA3F depolymerizes (collapses) the F-actin cytoskeleton by inactivating RhoA. As a consequence, the migration and contractility of these cells are inhibited. SEMA3F expression is regulated by the bHLH transcription factor, E47 (promotes expression) and Id2, a dominant negative of E47 (inhibits expression).