Guanine nucleotide-exchange factor (GEF) that activates CDC42 by exchanging bound GDP for free GTP. 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. Facilitates filopodia formation through the activation of CDC42. Interacts with CDC42. The DOCKER domain is necessary for the GEF activity. The DOCKER domain mediates interaction with activated CDC42 in conjunction with residues 66-126. 'Zizim' means 'spike' in Hebrew. Belongs to the DOCK family. Truncated N-terminus. Truncated N-terminus.