The anterior commissure (AC) is one of the important commissure projections in the brain that conveys information from one side of the nervous system to the other During development the axons from the anterior AC (aAC) and the posterior AC (pAC) course in the same dorsoventral plane and converge into a common fascicle for midline crossing Previously we reported that Tsukushi (TSM) a member of the secreted small leucine rich repeat proteoglycan family functions as a key coordinator of multiple pathways outside of cells through the regulation of an extracellular signaling network Here we show evidence that TSK is critical for the formation of the AC In mice lacking TSK the aAC and the pAC axons fall to cross the midline leading to an almost total absence of the AC in adult mice DiI labeling Indicated that the aAC axons grew out from the anterior olfactory nucleus and migrated along normal pathways but never crossed the midline Therefore we have uncovered a crucial role for TSK for AC formation in the mouse brain (C) 2010 Elsevier Inc All rights reserved