Your basis is zero. Because it was deferred compensation in the 401k (pre tax contributions from your wages) there was no basis when you transferred it to the traditional IRA. Hence, there was no basis when you moved it to the Roth IRA (which is the question TurboTax is asking- the taxable amount is Box 2a indicates this, but there are occasions that the issuer of the 1099-R does not know the entire situation.)
If you had basis, it would mean that you paid tax on the money at some point (which gets more difficult with time and more movement)- but in your case it is a clean move. Let me know if you have any other questions.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
