General and Administrative Expense
General and administrative expense consists primarily of salaries, consulting fees and other related costs, professional fees for legal services, accounting and tax services, insurance and travel expenses, as well as the stock-based compensation expense associated with equity awards to our employees, consultants, and directors.
General and administrative expense, excluding stock-based compensation expense, was approximately $5.1 million for the three months ended June 30, 2019, an increase of approximately $0.5 million from approximately $4.6 million for the same period in 2018. The increase was primarily due to an increase of approximately $0.9 million of employee related expenses and $0.2 million of rent expenses for our new office in South San Francisco, partially offset by a decrease of approximately $0.6 million of professional fees.
Stock-based compensation expense was approximately $(1.1) million for the three months ended June 30, 2019, a decrease of approximately $8.9 million from approximately $7.9 million for the same period in 2018. The change was primarily due to a $4.3 million one-time expense related to the departure and transition to consultant of one of our former officers in the second quarter of 2018 coupled with the reversal of previously recognized expense of $3.6 million related to forfeited awards from the departure of one of our former officers during the second quarter of 2019.
Comparison of the Six Months Ended June 30, 2019 and 2018
Collaboration Revenue
For the six months ended June 30, 2019 and 2018, collaboration revenue was approximately $7.0 million and $6.8, respectively, which included the recognition of deferred revenue and reimbursements in each case incurred under the Collaboration Agreement. The increase in revenue as compared to the same period in 2018 is a result of increased reimbursement activities, partially offset by lower deferred revenue being recognized as a result of changes in our estimates of the performance period associated with our Development Activities under the Collaboration Agreement.
Research and Development Expense
Research and development expense, excluding stock-based compensation expense, was approximately $35.6 million for the six months ended June 30, 2019, an increase of approximately $9.4 million from approximately $26.2 million for the same period in 2018. The increase was primarily due to an increase of approximately $7.5 million in research expenses for our Microbiome program due to increased headcount and launching of Phase 1b human clinical study for ABI-M201 and an increase of approximately $1.9 million in research expenses for our HBV-cure program.
Stock-based compensation expense was approximately $5.8 million for the six months ended June 30, 2019, a decrease of approximately $0.4 million from approximately $6.2 million for the same period in 2018.
General and Administrative Expense
General and administrative expense consists primarily of salaries, consulting fees and other related costs, professional fees for legal services, accounting and tax services, insurance and travel expenses, as well as the stock-based compensation expense associated with equity awards to our employees, consultants, and directors.
General and administrative expense, excluding stock-based compensation expense, was approximately $10.8 million for the six months ended June 30, 2019, an increase of approximately $2.0 million from approximately $8.8 million for the same period in 2018. The increase was primarily due to an increase of approximately $0.1 million of professional fees, $1.3 million of employee related expenses, and $0.6 million of rent expenses for our new office in South San Francisco.
Stock-based compensation expense was approximately $2.8 million for the six months ended June 30, 2019, a decrease of approximately $6.7 million from approximately $9.5 million for the same period in 2018. The decrease was primarily due to a $4.3 million one-time expense related to the departure and transition to consultant of one of our former officers in 2018 coupled with the reversal of previously recognized expense of $3.6 million related to forfeited awards resulting from the departure of one of our former officers during the period.