As of June 30, 2024, we had a security deposit of $5.9 million related to this lease in the form of a cash collateralized letter of credit issued with UBS Bank USA, which is classified as long-term restricted cash on our balance sheet at June 30, 2024.
Note 14. Income Taxes
Income tax expense was $6.4 million for the three months ended June 30, 2024, compared to $3.0 million for the three months ended June 30, 2023. The $3.4 million increase was primarily due to a decrease in stock compensation deduction. Income tax expense was $13.8 million for the six months ended June 30, 2024, compared to $8.2 million for the six months ended June 30, 2023. The increase was primarily due to a decrease in stock compensation deduction.
The effective tax rate for the three and six months ended June 30, 2024 was less than the U.S. statutory rate of 21% due to a forecasted Foreign Derived Intangible Income deduction, Federal research and development tax credits and a favorable discrete item related to equity compensation that reduces the annual tax rate.
The deferred income taxes of $55.4 million and $53.4 million as of June 30, 2024 and December 31, 2023, respectively, reflect the net tax effect of temporary differences between the carrying amount of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, as well as the tax effect of carryforwards. As of June 30, 2024, we have recorded a $11.3 million valuation allowance in the U.S. against certain tax credits due to the uncertainty of their realization. Realization of our net deferred tax assets is dependent on future taxable income. We believe it is more likely than not that such assets will be realized; however, ultimate realization could be impacted by market conditions and other variables not known or anticipated at this time.
Note 15. Concentration of Risk
For the three months ended June 30, 2024, one customer accounted for 11.5% of total revenue. For the three months ended June 30, 2023, no individual customer accounted for greater than ten percent of total revenue.
For the six months ended June 30, 2024, no individual customer accounted for greater than ten percent of total revenue. For the six months ended June 30, 2023, two customers accounted for 11.6% and 11.1% of total revenue, respectively.
At June 30, 2024, one customer accounted for 11.3% of accounts receivable. At December 31, 2023, one customer accounted for 12.2% of accounts receivable.
Note 16. Share Repurchase
In February 2022, our Board of Directors approved stock repurchases of up to $100 million of our common stock. In August 2023, our Board of Directors approved additional funding of $200 million for our stock repurchase program, to be available on full utilization of the $100 million repurchase funding approved in February 2022. During the six months ended June 30, 2024, we repurchased 0.3 million shares at an average cost of $114.07 per share. The timing and actual number of any additional shares to be repurchased under this program will depend on various factors including price, corporate and regulatory requirements, alternative investment opportunities and other market conditions.
Repurchased shares are accounted for when the transaction is settled and returned to the status of authorized but unissued shares. Accordingly, on our balance sheet, the repurchase price is deducted from common stock par value and from additional paid-in capital for the excess over par value. If additional paid-in capital has been exhausted, the excess over par value is deducted from retained earnings. Direct costs incurred to acquire the shares are included in the total cost of the shares.
Note 17. Contingencies
(a) Litigation
We are from time to time a party to litigation that arises in the normal course of our business operations. We are not presently a party to any litigation that we believe might have a material adverse effect on our business operations.