Guanine nucleotide-exchange factor (GEF) that activates CDC42 by exchanging bound GDP for free GTP (PubMed:15710388, PubMed:16968698, PubMed:25851601). Required for marginal zone (MZ) B-cell development, is associated with early bone marrow B-cell development, MZ B-cell formation, MZ B-cell number and marginal metallophilic macrophages morphology (PubMed:25729399). Facilitates filopodia formation through the activation of CDC42 (PubMed:22494997). Interacts with CDC42. Expressed in spleen, thymus, mesenteric lymph nodes (MLN), bone marrow and peripheral blood lymphocytes. Enriched in B-cells from germinal centers. Expressed in B-, T- and dendritic cells as well as Purkinje cells (PubMed:22494997, PubMed:25851601). In spleen, expression is down-regulated in aged mice. In dendritic cells, the expression is up-regulated by LPS and anti-Fc-gamma receptor. The DOCKER domain is necessary for the GEF activity (PubMed:25851601, PubMed:15710388). The DOCKER domain mediates interaction with activated CDC42 in conjunction with residues 66-126 (PubMed:16968698). Knockout mice are viable and fertile (PubMed:25729399). They have higher percentage of early bone marrow B-cells, but a reduced fraction of marginal zone B-cells. Their percentage of thymic CD4(+) T-cells is increased and they show an altered of morphologymarginal metallophilic macrophages (PubMed:25729399). 'Zizim' means 'spike' in Hebrew. Belongs to the DOCK family.