5. SIGNIFICANT CONCENTRATIONS

We recognize revenue on collaborations in the U.S. and abroad and on products sold solely in the U.S. For the year ended December 31, 2019, our three specialty distributors as well as Aclaris, Grifols, Celgene and Kissei (see Note 2) accounted for 74%,  9%,  8%,  6% and 3% of our total revenues, respectively. For the year ended December 31, 2018, Kissei and our three specialty distributors (see Note 2) accounted for 69% and 31% of our total revenues, respectively.  For the year ended December 31, 2017, BerGenBio and Celgene accounted for 74% and 26% of our total revenues, respectively. As of December 31, 2019 and 2018, 100% of our accounts receivables are from four customers and one collaboration partner.