revenue to increase in absolute dollars as, and to the extent, our sales volume grows. Any delay in volume growth, whether due to macroeconomic pressures or otherwise, could lead to additional charges to excess and obsolete inventory.
Amortization of Intangible Assets
Amortization of intangible assets relates to the amortization of capitalized milestone amounts paid to Aroa related to license fees or commercialization rights after future economic benefit has been established for a product. These capitalized milestone amounts relate to regulatory clearances, the receipt of certain supply quantities of product, and amounts based upon aggregate net sales thresholds within a specified territory, and are amortized over the remaining useful life of the intellectual property.
Gross Profit and Gross Margin
Our gross profit is calculated by subtracting our cost of revenue and amortization of intangible assets from our revenue. We calculate our gross margin percentage as our gross profit divided by our revenue. Our gross margin has been, and we expect it will continue to be, affected by a variety of factors, including sales volume, royalties and inventory excess and obsolescence costs. Our gross profit may increase to the extent our revenue grows.
Sales and Marketing Expenses
Sales and marketing expenses consist of commercial activities related to the sale of our products, along with the salaries and related benefits, including sales commissions and stock-based compensation for employees focused on these efforts. Other significant sales and marketing expenses include costs incurred with post-market clinical studies, conferences and trade shows, promotional and marketing activities, market research, as well as travel and training expenses.
We expect future sales and marketing expenses will primarily depend on our ability to drive operational leverage and efficiencies from our expanded commercial organization. We expect our sales and marketing expenses to continue to decrease as a percentage of revenue, as and to the extent, our revenue grows.
General and Administrative Expenses
General and administrative expenses consist primarily of salaries and related benefits, including stock-based compensation for personnel in executive, finance, information technology and administrative functions. General and administrative expenses also include professional service fees for legal, accounting, consulting, investor and public relations, insurance costs and direct and allocated facility-related costs.
We expect future general and administrative expenses will primarily depend on our ability to efficiently execute on our growth initiatives. We expect our general and administrative expenses to decrease as a percentage of revenue primarily as, and to the extent, our revenue grows.
Research and Development Expenses
Research and development expenses consist primarily of product research, engineering, product development, regulatory compliance and clinical development. These expenses include salaries and related benefits including stock-based compensation, for employees focused on these efforts, consulting services, costs associated with our preclinical studies, costs incurred with our manufacturing partner under development agreements related to technology transfer, costs incurred from license agreements with no alternative future uses, laboratory materials and supplies and an allocation of related facilities costs. We expense research and development costs as they are incurred.
We expect future research and development expenses will primarily depend on our ability to efficiently develop new products, enhance existing products and conduct research to generate clinical data in support of new or expanded indications for our products. We expect research and development expenses as a percentage of revenue to vary over time depending on the level and timing of new product development and clinical trial initiatives.