Guanine nucleotide-exchange factor (GEF) that activates CDC42 by exchanging bound GDP for free GTP (PubMed:25851601). Overexpression induces filopodia formation (By similarity). Homodimer. Interacts preferentially with nucleotide-depleted CDC42 (By similarity). Associated with membranes. Expressed in lung (PubMed:25729399, PubMed:22494997). Also detected in Peyers patches, thymus, brain and lymph nodes (PubMed:22494997). Expressed in Purkinje cells (PubMed:25851601). The DOCKER domain is necessary and sufficient for the GEF activity. 'Zizim' means 'spike' in Hebrew. Belongs to the DOCK family. Truncated N-terminus. Extended N-terminus.