Guanine nucleotide-exchange factor (GEF) that activates CDC42 and RAC1 by exchanging bound GDP for free GTP. Essential for dendritic spine morphogenesis in Purkinje cells and in hippocampal neurons, via a CDC42-mediated pathway. Sustains B-cell lymphopoiesis in secondary lymphoid tissues and regulates FCER2/CD23 expression. Expressed at low level in brain and lung. Isoform 1 is enriched in normal T-cells, isoform 3 is enriched in normal B-cells and chronic lymphocytic leukemia (CLL) B-cells. Isoforms 1 and 2 are up-regulated in response to IL4 in B-cells but not T-cells. The DOCKER domain may mediate some GEF activity. 'Zizim' means 'spike' in Hebrew. Belongs to the DOCK family. Truncated N-terminus. Extended N-terminus. Truncated N-terminus. Truncated N-terminus.