[P2.40]: A requirement for the forkhead protein foxp1 and hox proteins in the columnar organization of spinal motor neurons

cyclin EmRNA level, increased cyclin/CDKactivitymight account for the increased cell proliferation. To characterize effects on cortex structure, we performed stereological analysis at E18.5 and found a 15% increase in total volumeand26% increase in total cells.Whileno change in cell density was observed, the depth of layers I and VIwas increased (39% and 19%, respectively). Furthermore, using the layer VI specific marker Tbr1, we found a 40% increase in the mutants at E18.5 and a 2-fold increase in protein by western blotting. Our observations suggest that p57 is a critical regulator of cortical precursor proliferation, andmay be determinant of laminar-specific cell production during cortical neurogenesis.