initial licensing fee of $2.0 million. In connection with the amendment to the REGENXBIO SMA License, we paid or are required to pay additional fees of $80.0 million upon entry into the amendment, $30.0 million on the first anniversary of the effective date of the amendment and $30.0 million on the second anniversary of the effective date of the amendment. We are also required to pay REGENXBIO annual maintenance fees, up to $120.0 million in sales-based milestone fees for all REGENXBIO SMA products covered by the REGENXBIO SMA License, or REGENXBIO SMA Licensed Products; mid-single to low-double digit royalty percentages on net sales of REGENXBIO SMA Licensed Products using AAV9, subject to reduction in specified circumstances; a low-double digit royalty percentage on net sales of REGENXBIO SMA Licensed Products using an AAV vector other than AAV9, subject to reduction in specified circumstances; and lower mid-double digit percentages of any sublicense fees we receive from sublicensees for the licensed patent rights. As of December 31, 2017, we have paid $3.4 million under the REGENXBIO SMA License, which includes $1.3 million in aggregate milestone payments. We paid $80.0 million in connection with the amendment to the REGENXBIO SMA License in January 2018.
Asklepios Biopharmaceutical, Inc.
In May 2015, we entered into a non-exclusive, worldwide license agreement with Asklepios Biopharmaceutical, Inc., or AskBio, under certain patents and patent applications, for the use of AskBio’s self-complementary AAV genome technology for the treatment of SMA in humans, or the AskBio License. Under the AskBio License, we paid AskBio a one-time upfront license fee of $1.0 million, payable across stipulated milestones. We are also required to pay ongoing annual maintenance fees, up to a total of $0.6 million in clinical development milestone payments and up to a total of $9.0 million in commercial milestone payments. Under the terms of the AskBio License, we are required to pay AskBio annual tiered royalties on net sales of any products covered by the AskBio License, on a country-by-country basis, starting at percentages in the low-single digits and increasing to mid-single digits. These royalty rates are subject to potential reduction in specified circumstances, including, in the event we exercise our option to make a specified one-time royalty option fee payment to AskBio. We must also pay AskBio a low-double digit percentage of all consideration we receive from any sublicense of the licensed technology. Through December 31, 2017, we have paid the $1.0 million upfront license fee owed under the AskBio License and $0.6 million for a clinical development milestone payment.
Preclinical Programs for Rett Syndrome and ALS
We have also entered into three license agreements relating to our planned new programs for Rett syndrome and ALS.
Effective June 7, 2017, we entered into an exclusive license agreement with REGENXBIO under certain patents and patent applications owned by the Trustees of the University of Pennsylvania and licensed to REGENXBIO, for the development and commercialization of products to treat Rett syndrome and genetic ALS using the AAV9 vector, or the REGENXBIO Rett and ALS License. Under the REGENXBIO Rett and ALS License, REGENXBIO granted us an exclusive, worldwide license under the licensed patent rights to make, have made, use, import, sell and offer for sale any products covered by the REGENXBIO Rett and ALS License, or the REGENXBIO Rett and ALS licensed products, for the treatment of (i) Rett syndrome in humans by in vivo gene therapy using AAV9 delivering the gene encoding for methyl CpG binding protein 2, and (ii) ALS caused by SOD1 mutation in humans by in vivo gene therapy using AAV9 delivering the gene encoding for SOD1, subject to certain rights reserved by REGENXBIO and its licensors. The patent rights exclusively in-licensed include an issued United States patent, which expires in 2026. We have the right to sublicense the licensed technology to third parties subject to certain conditions as specified in the REGENXBIO Rett and ALS License. Under the REGENXBIO Rett and ALS License we grant a non-exclusive, worldwide, royalty-free, transferable, sublicenseable, irrevocable, perpetual license back to REGENXBIO to (a) use any patentable modifications and improvements to the licensed technology that we or our affiliates or sublicensees develop, or licensed back improvements, and (b) practice the licensed back improvements in connection with AAV9 outside of our fields of use.