Similar to most members of the Rho family of guanine-nucleotide binding proteins, Rac1 is a small GTPase (Mw ∼ 21 kDa) that cycles between active GTP-bound and inactive GDP-bound states, and is the most prominently expressed Rac GTPase in lung cancer (Caino et al., 2012). The gene discussed is RAC1; the disease is lung cancer.