User blog comment:Aleksandr the Great/Prothean Squadmate is Officially DLC/@comment-24.131.37.97-20120228031020/@comment-5503561-20120228190703

EDI mentions several generations of cloned Protheans came before full Collector transformation. Also, it would be logical to preserve atleast one member of a species as a control subject, if for nothing else than to have the DNA for further cloning.