RNA recognition motif (RRM) found in the cell surface Ecto-NOX disulfide-thiol exchanger (ECTO-NOX or ENOX) proteinsThis subgroup corresponds to the conserved RNA recognition motif (RRM) in ECTO-NOX proteins (also termed ENOX), comprising a family of plant and animal NAD(P)H oxidases exhibiting both, oxidative and protein disulfide isomerase-like, activities. They are growth-related and drive cell enlargement, and may play roles in aging and neurodegenerative diseases. ENOX proteins function as terminal oxidases of plasma membrane electron transport (PMET) through catalyzing electron transport from plasma membrane quinones to extracellular oxygen, forming water as a product. They are also hydroquinone oxidases that oxidize externally supplied NADH, hence NOX. ENOX proteins harbor a di-copper center that lack flavin. ENOX proteins display protein disulfide interchange activity that is also possessed by protein disulfide isomerase. In contrast to the classic protein disulfide isomerases, ENOX proteins lack the double CXXC motif. This family includes two ENOX proteins, ENOX1 and ENOX2. ENOX1, also termed candidate growth-related and time keeping constitutive hydroquinone [NADH] oxidase (cCNOX), or cell proliferation-inducing gene 38 protein, or Constitutive Ecto-NOX (cNOX), is the constitutively expressed cell surface NADH (ubiquinone) oxidase that is ubiquitous and refractory to drugs. ENOX2, also termed APK1 antigen, or cytosolic ovarian carcinoma antigen 1, or tumor-associated hydroquinone oxidase (tNOX), is a cancer-specific variant of ENOX1 and plays a key role in cell proliferation and tumor progression. In contrast to ENOX1, ENOX2 is drug-responsive and harbors a drug binding site to which the cancer-specific S-peptide tagged pan-ENOX2 recombinant (scFv) is directed. Moreover, ENOX2 is specifically inhibited by a variety of quinone site inhibitors that have anticancer activity and is unique to the surface of cancer cells. ENOX proteins contain many functional motifs.