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 (PubMed:25851601). Sustains B-cell lymphopoiesis in secondary lymphoid tissues and regulates FCER2/CD23 expression (PubMed:27502165). Expressed in brain, lung, spleen, mesenteric lymph nodes (MLN) and thymus (PubMed:15710388, PubMed:25729399). Expressed by B and T splenocytes (PubMed:21514340). In brain, expressed by Purkinje cells during postnatal development and hippocampal neurons (PubMed:25851601). At early postnatal stages, expressed broadly in the cerebellum and from P15 the expression becomes restricted to Purkinje cells. Isoform 4 (but not isoform 1) is highly induced by IL4 in B splenocytes, but not T splenocytes. The DOCKER domain may mediate some GEF activity. Knockout mice are viable and fertile (PubMed:25729399, PubMed:27502165). They show decreased numbers of B-cells in spleen, both follicular B-cells and marginal zone B-cells, and in peripheral blood, but not in bone marrow (PubMed:25729399, PubMed:27502165). Their percentage of splenic CD8(+) T-cells is decreased comparing to wild types (PubMed:25729399). 'Zizim' means 'spike' in Hebrew. Belongs to the DOCK family. Truncated N-terminus.