sentence: Clinical trials were conducted using catheters releasing recombinant GDNF protein ( Nutt et al ., 2003 ; Lang et al ., 2006 ) as well as rAAV serotype 2 ( rAAV2 ) - mediated delivery of the NRTN ( Marks et al ., 2010 ; Warren Olanow et al ., 2015 ) or GDNF cDNA ( https://clinicaltrials.gov/ct2/show/NCT01621581?term=AAV2-GDNF&rank=1 ) .