Guanine nucleotide-exchange factor (GEF) that activates CDC42 by exchanging bound GDP for free GTP. Overexpression induces filopodia formation. Homodimer (Probable). Interacts preferentially with nucleotide-depleted CDC42. Associated with membranes. Widely expressed, with highest expression in heart and placenta. Expressed at intermediate level in kidney, brain, lung and skeletal muscle. The DOCKER domain is necessary and sufficient for the GEF activity. 'Zizim' means 'spike' in Hebrew. Produced by exon skipping that results in a frameshift. Belongs to the DOCK family.