We derive our revenues from tests based on units reported to customers—tests delivered with a result. All reported units are either accessioned in our laboratory or processed outside of our laboratory. As noted in the section titled “Overview” above, the number of tests that we process is a key metric as it tracks overall volume growth. During the three months ended September 30, 2021, total reported units were approximately 373,100, comprised of approximately 360,700 tests reported in our laboratory. Comparatively, during the three months ended September 30, 2020, total reported units were approximately 238,600, comprising of approximately 226,700 tests reported in our laboratory.
Product Revenues
During the three months ended September 30, 2021, product revenues increased by $57.4 million, or 61.5% compared to the three months ended September 30, 2020, as a result of the continued revenue growth from test volumes.
Licensing and Other Revenues
Licensing and other revenues increased by $2.6 million, or 53.7%, during the three months ended September 30, 2021 when compared to the three months ended September 30, 2020. The increase in revenue was primarily due to an increase in revenues recognized from our collaborative agreements.
Cost of Product Revenues
During the three months ended September 30, 2021, cost of product revenues increased compared to the three months ended September 30, 2020 by approximately $30.0 million, or 63.9%, due to a $10.0 million increase in third-party fees, higher costs related to inventory consumption of $2.8 million driven by an increase in accessioned tests, a $1.8 million increase in shipping related charges, and a $15.4 million increase in labor and overhead costs driven by headcount growth and product support.
Cost of Licensing and Other Revenues
Cost of licensing and other revenues for the three months ended September 30, 2021, when compared to the three months ended September 30, 2020, decreased by $0.4 million, or 8.4%, due to a decrease in labor and overhead costs related to license and development support.
Research and Development
Research and development expenses during the three months ended September 30, 2021, increased by $72.0 million, or 272.6%, when compared to the three months ended September 30, 2020. The increase was driven by a $15.5 million increase in salary and related expenditures primarily due to headcount growth, which includes a $3.6 million increase in stock-based compensation expense, an increase of $8.4 million of consulting costs, a $7.9 million increase of costs related to clinical studies to support our new product offerings, a $35.6 million write-off of the acquired in-process research and development asset, $1.2 million of expenses related to acquisition-related milestones and a $3.4 million increase in facilities, office and other costs.
Selling, General and Administrative
Selling, general and administrative expenses increased by $52.8 million, or 69.8%, during the three months ended September 30, 2021 compared to the three months ended September 30, 2020. The increase was attributable to an increase of $29.7 million in salary and related expenditures primarily due to headcount growth, which includes a $7.0 million increase in stock-based compensation expense, a $5.1 million increased in marketing expenses, a $4.1 million increase in travel related costs, a $8.1 million increase in consulting and legal fees, a $1.6 million increase in hardware and software licenses, and a $4.2 million increase from business insurance and other administrative costs.