5. Property and Equipment

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Fortress' depreciation expense for the three months ended September 30, 2024 and 2023 was approximately $0.1 million and $0.4 million, respectively, and for the nine months ended September 30, 2024 and 2023 was approximately $0.9 million and $1.9 million, respectively. Fortress’ depreciation expense is recorded in both research and development expense and general and administrative expense in the condensed consolidated statement of operations.

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Impairment of Long-Lived Assets

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During the nine months ended September 30, 2024, Mustang concluded it had a triggering event requiring assessment of impairment for certain leasehold improvements and the related right of use asset. Mustang assessed the carrying value of the asset group consisting of the leasehold improvements and right-of-use asset in accordance with ASC 360, given the significant changes to Mustang’s operations, operating cash and the repurchase of equipment. The assessment of the recoverability of the asset group concluded that there was impairment on the carrying value of the asset group of approximately $2.6 million, which was allocated on a pro rata basis using the relative carrying amounts of the assets.  Approximately $2.2 million of the impairment loss was allocated to leasehold improvements, with the remaining $0.4 million allocated to the right-of-use asset.