CRABP1 and neuroblastoma: Specifically, cellular retinoic acid binding protein (CRABP) was selected as the first target protein because (i) CRABP-II induces migration of neuroblastoma cells independently of its endogenous ligand, all-trans retinoic acid (ATRA, 8) [62]; (ii) no direct CRABP inhibitor has been reported; (iii) CRABPs bind strongly to ATRA (the Kd values of CRABP-I and CRABP-II are 5 and 60 nM, respectively [63]); and (iv) sufficient information is available about the effect of linker position on the binding affinity, as discussed below.