Revenue Recognition We recognize revenue when our customers obtain control of the promised goods or services, in an amount that reflects the consideration which we expect to receive in exchange for those goods or services. To account for our revenue arrangements, we perform the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) we satisfy our performance obligations. For contracts determined to be within the scope of ASU 2014 - 09, Revenue from Contracts with Customers (Topic 606 ) (ASU 2014 - 09 or the Topic 606 ), we assess the goods or services promised within each contract for the purpose of identifying them as performance obligations. We must apply judgement in assessing whether each promised good or service is distinct. If a promised good or service is not distinct, we will combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their estimated fair value, which requires significant judgment. Variable consideration, which is estimated using the expected value method or the most likely amount method, is included in the transaction price only if, in our judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. For arrangements that include development, regulatory or sales milestone payments, we evaluate whether the milestones are probable of being reached and estimate the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within our control or the licensees control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. We then recognize as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. The following table summarizes our revenue streams from contracts with customers for the three and six months ended June 30, 2019 and 2018 (in thousands): Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Product sales $ 270 $ - $ 270 $ - Royalty income 3,593 3,530 7,754 6,588 License and other revenue 6,103 348 6,223 479 Total revenue $ 9,966 $ 3,878 $ 14,247 $ 7,067 Product sales  represent revenue from sales of AZEDRA directly to hospitals. Our performance obligations are to provide AZEDRA based on sales orders from hospitals, which have been verified by our commercial team as properly credentialed to receive, handle, administer and dispose of radioactive materials. We recognize revenue after the customer takes title and has obtained control of the product. Our contracts with hospitals stipulate that product is shipped free on board destination. We invoice hospitals after the products have been delivered and invoice payments are generally due within 60 days of invoice date. We record sales to hospitals based on AZEDRAs list price per mCi and the amount prescribed based upon a dosimetric assessment of each patient. We record product sales net of any variable consideration due to rebates and discounts provided under governmental and other programs, and other sales-related deductions, such as product returns and copay assistance programs. Shipping and handling costs associated with finished goods delivered to customers are recorded as a selling expense. Royalty income  represents revenue from the sales-based royalties under our intellectual property licensing arrangements and is recognized upon net sales of the licensed products. License and o ther revenue  represents revenue from upfront payments (fixed consideration) and development and sales milestones, sublicense payments, support and service payments and sales-based bonus payments (variable consideration) under our licensing or software arrangements. The fixed consideration will be recognized as revenue at the time when the transfer of know-how is completed. The variable consideration will be estimated using the most likely amount method and recognized only when we have a high degree of confidence that revenue will not be reversed in a subsequent reporting period. The other revenue also includes revenue from product sales of research reagents, that is recognized upon shipment to the end customer (i.e. control of the product is deemed to be transferred). We had receivable contract balances of $10.6 million and $3.8 million as of June 30, 2019 and December 31, 2018, respectively, primarily related to the royalty revenue and upfront and milestone payments under our partnerships, which are included in license and other revenue (see Note 5 . Accounts Receivable ).