Read the following text from the article concerning oocyte s…

Read the following text from the article concerning oocyte stem cells (OSCs):   “…any consideration of a possible clinical utility of OSCs requires firm evidence that oocyte-producing cells exist in ovaries of reproductive-age women. As a first step toward this goal, we assessed a protocol to obtain OSCs from adult mouse ovaries. This protocol relies on the immunological detection of a putative cell-surface variant of Ddx4 (DEAD box polypeptide 4…” Ddx4 is a(n) _____________.