Review your notice carefully to confirm that the penalty is only for underpayment of estimated tax (see the information provided by tagteam above) and not also an accuracy-related penalty for substantial understatement of tax. You stated that you received a CP2000 and that you left an item of income off of the tax return. The two penalties have different criteria for relief.
See here for more information from IRS about the accuracy-related penalty for substantial understatement of tax.
Your CP2000 will have specific instructions on how to protest any of the changes proposed.
@StoneCold
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