sentence: From them , genetic therapy directed to increase the expression of GDNF and NRTN alone or combined with other NFs have shown promising results.The study of GDNF both in vitro and in vivo using rodent ( Rosenblad et al ., 1998 ; Sullivan et al ., 1998 ; Georgievska et al ., 2002 ) and monkey models of PD ( Gash et al ., 1996 ; Miyoshi et al ., 1997 ; Palfi et al ., 2002 ; Eberling et al ., 2009 ; Su et al ., 2009 ; Redmond et al ., 2013 ) , revealed potential therapeutic effects that encouraged its clinical investigation .