30, 2023 and 2022, respectively. For the six months ended June 30, 2023 and 2022, net product sales in the United States were $120.3 million and $105.4 million, respectively, consisting solely of Emflaza. During the six months ended June 30, 2023, three countries, the United States, Russia, and Brazil, accounted for at least 10% of the Company’s net product sales, representing $120.3 million, $63.7 million, and $48.9 million of net product sales, respectively. During the six months ended June 30, 2022, three countries, the United States, Russia, and Brazil, accounted for at least 10% of the Company’s net product sales, representing $105.4 million, $28.9 million, and $32.5 million of net product sales, respectively. For the six months ended June 30, 2023 and 2022, the Company had a total of two and two distributors, respectively, that each accounted for over 10% of the Company's net product sales.
As of June 30, 2023 and December 31, 2022, the Company does not have a contract liabilities balance related to net product sales, and has not made significant changes to the judgments made in applying ASC Topic 606.
Collaboration and Royalty revenue
In November 2011, the Company and the SMA Foundation entered into the SMA License Agreement with Roche. Under the terms of the SMA License Agreement, Roche acquired an exclusive worldwide license to the Company’s SMA program.
Under the SMA License Agreement, the Company is eligible to receive additional payments from Roche if specified events are achieved with respect to each licensed product, including up to $135.0 million in research and development event milestones, up to $325.0 million in sales milestones upon achievement of specified sales events, and up to double digit royalties on worldwide annual net sales of a commercial product.
The SMA program currently has one approved product, Evrysdi, which was approved in August 2020 by the FDA for the treatment of SMA in adults and children two months and older. As of June 30, 2023, the Company does not have any remaining research and development event milestones that can be received. The remaining potential sales milestones that can be received is $250.0 million.
For the three months ended June 30, 2023 and 2022, the Company did not recognize collaboration revenue related to the SMA License Agreement with Roche. For the six months ended June 30, 2023 and 2022, the amounts recognized for the collaboration revenue related to the SMA License Agreement with Roche were immaterial.
In addition to research and development and sales milestones, the Company is eligible to receive up to double-digit royalties on worldwide annual net sales of a commercial product under the SMA License Agreement. For the three and six months ended June 30, 2023, the Company has recognized $36.9 million and $67.7 million of royalty revenue related to Evrysdi, respectively. For the three and six months ended June 30, 2022, the Company has recognized $21.8 million and $40.7 million of royalty revenue, respectively, related to Evrysdi.
Manufacturing Revenue
For the three and six months ended June 30, 2023, the Company recognized $2.4 million and $4.4 million of manufacturing revenue, respectively, related to the production of plasmid DNA and AAV vectors for gene therapy applications for external customers. No manufacturing revenue was recognized in the three and six months ended June 30, 2022. The Company has not made significant changes to the judgments made in applying ASC Topic 606 for the three and six months ended June 30, 2023 and 2022.
The Company does not have a contract liabilities balance for the period ended June 30, 2023. As of June 30, 2023, the Company has contract assets of $0.4 million related to the production of plasmid DNA and AAV vectors for gene therapy applications for external customers, which is recorded within prepaid expenses and other current assets on the consolidated balance sheet. As of December 31, 2022, the contract liabilities balance related to plasmid DNA and AAV production for external customers was $1.4 million, which is recorded within deferred revenue on the consolidated balance sheet. The Company did not have any contract assets for the period ending December 31, 2022. For the three and six months ended June 30, 2023, the Company recognized revenue of $0.2 million and $1.6 million, respectively, related to the amounts included in the contract liability balance at the beginning of the period.