Guanine nucleotide-exchange factor (GEF) that activates CDC42 by exchanging bound GDP for free GTP. Overexpression induces filopodia formation (By similarity). Homodimer. Interacts preferentially with nucleotide-depleted CDC42 (By similarity). Associated with membranes. The DOCKER domain is necessary and sufficient for the GEF activity. 'Zizim' means 'spike' in Hebrew. Belongs to the DOCK family.