Migration of oligodendrocyte precursor cells (OPCs) is essential for formation and repair of myelin within the CNS. OPCs migrate towards sources of platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) and respond to guidance factors such as netrin. Their migration is also stimulated by cell adhesion molecules such as polysialic acid neural cell adhesion molecule (PSA-NCAM); however, the prevailing view has been that these merely `grease the wheels' by reducing adhesion forces rather affect the direction the cells choose. Jozsef Kiss and co-workers now show that this is not the case (see p. 93). They show that disrupting the PSA-NCAM PSA tail or removing it with endoneuraminidase prevents cells from responding correctly to PDGF in Boyden and Dunn chemotaxis chambers. The cells polarize and migrate but do not move in the right direction, instead making random turns. Intriguingly, the effect is specific to PDGF – loss of PSA does not affect the response to FGF-2. PSA-NCAM thus appears to be important for establishing the directionality of PDGF-directed motility rather than for locomotion itself.