Deferred Tax Assets and Valuation Allowance
Deferred tax assets reflect the tax effects of net operating losses (“NOLs”), tax credit carryovers, and temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. The most significant item of our deferred tax assets is derived from our Federal NOLs. We have approximately $167.8 million gross Federal NOLs at December 31, 2019 (of which approximately $160.2 million was generated prior to January 1, 2018). Because we believe the ability for us to use these NOLs generated prior to January 1, 2018 to offset any future taxable income is severely limited as prescribed under Internal Revenue Code (“IRC”) Section 382, we had estimated and recorded an amount for the likely limitation to our deferred tax asset in the fourth quarter of 2017, thereby reducing the aggregate estimated benefit of the Federal NOLs available to us of approximately $1.0 million at December 31, 2017. We believe the gross Federal NOL benefit we generated prior to January 1, 2018 to offset taxable income is less than $150 thousand annually. As prescribed under Internal Revenue Code, any unused Federal NOL benefit from the annual limitation can be accumulated and carried forward to the subsequent year and will expire if not used in accordance with the NOL carried forward term of 20 years or 2037, if generated before 2018 and Federal NOLs generated after 2017 can be carried forward indefinitely. Future common stock transactions, such as the exercise of common stock purchase warrants or the conversion of debt into common stock, may cause another qualifying event under IRC 382 which may further limit our utilization of our NOLs.
The components of our deferred tax assets are as follows (in thousands):
The realization of deferred income tax assets is dependent upon future earnings, if any, and the timing and amount of which may be uncertain. A valuation allowance is required against deferred income tax assets if, based on the weight of available evidence, it is more likely than not that some or all of the deferred income tax assets may not be realized. At both December 31, 2019 and 2018, all our remaining net deferred income tax assets were offset by a valuation allowance due to uncertainties with respect to future utilization of NOL carryforwards. If in the future it is determined that additional amounts of our deferred income tax assets would likely be realized, the valuation allowance would be reduced in the period in which such determination is made and an additional benefit from income taxes in such period would be recognized.
Table:
 |  | December 31,
 | 2019 | 2018
Deferred tax assets: |  | 
Estimated future value of NOLs |  | 
- Federal | $2,622 | $2,174
- State | 869 | 862
Research and development tax credits |  | 
- Federal | 1,207 | 1,184
- State | 8 | -
Share-based compensation | 72 | 71
Other, net | 177 | 151
Total deferred taxes | 4,955 | 4,442
Valuation allowance | (4,955) | (4,442)
Net deferred tax assets | $- | $-


Question: Why are deferred income tax assets offset in 2018 and 2019?
Answer: At both December 31, 2019 and 2018, all our remaining net deferred income tax assets were offset by a valuation allowance due to uncertainties with respect to future utilization of NOL carryforwards.