26 USC 6719 - Failure to register or reregister

(a) Failure to register or reregister 
Every person who is required to register or reregister under section 4101 and fails to do so shall pay a penalty in addition to the tax (if any).
(b) Amount of penalty 
The amount of the penalty under subsection (a) shall be
(1) $10,000 for each initial failure to register or reregister, and
(2) $1,000 for each day thereafter such person fails to register or reregister.

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(c) Reasonable cause exception 
No penalty shall be imposed under this section with respect to any failure if it is shown that such failure is due to reasonable cause.