Rho guanine nucleotide exchange factor 39 (ARHGEF39) is a new member of the Dbl-family of guanine nucleotide exchange factors (GEFs), which are pivotal catalysts of GTPases and are markedly associated with tumor metastasis (Rossman et al., 2005; Goicoechea et al., 2014; Miller et al., 2014; Hennig et al., 2015). The gene discussed is MCF2; the disease is neoplasm.