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You can plead the fifth amendment, however the DA need only grant you immunity and then you can be compelled to testify. Many witnesses do not wish to testify for the prosecution to a crime or portion of a crime they witnessed... that is a very natural feeling. However, the state has placed you under subpoena and you must comply with that subpoena or you could find yourself in trouble as well. Further, before I'd take the fifth I would ask to speak to an attorney and let them evaluate the entire case to determine if you are at risk of jeopardizing yourself. You don't necessarily need a lawyer with you when you testify... but I would have a lawyer review the case before and give an opinion whether or not you could be facing potential criminal liability. Your attorney may even be able to arrange immunity before you show up to court.
The Fifth Amendment to the U.S. Constitution protects you from being compelled into making incriminating statements against yourself. Therefore, if you were facing charges or could potentially face charges the state could not force you to testify or answer any questions. However, if the state grants you immunity from your testimony then your Fifth Amendment rights evaporate and you must testify.
In short... you must comply with a subpoena.
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