Figure 1: Concept of chemotactic “tug of war” gradient between BM and PB to explain mobilization and homing of HSPCs. Panel A – It has been postulated that SDF-1 gradient between BM and PB regulates trafficking of HSPCs (homing vs. mobilization). In steady state conditions this gradient should be in balance. As proposed during mobilization increase in plasma SDF-1 level directs egress of HSPCs and vice versa increase in BM SDF-1 level after conditioning for transplantation was supposed to direct engraftment of HSPCs. Panel B – Novel model in which bioactive lipids S1P and C1P modulate function of SDF-1 in mobilization and homing.