Legal analysis: sign the overdue bill when it is written, as long as the debtor signs it. It is invalid for anyone to sign on behalf of it. The IOU is valid as long as the debtor signs it, and the IOU is valid as long as the borrower signs it. However, the reasons for owing money are illegal, and the signer's signature is not protected by law, such as owing money for whoring, giving money for drugs, and stealing money.

Legal basis: Civil Code of People's Republic of China (PRC).

Article 667 A loan contract is a contract in which the borrower borrows money from the lender, repays the loan at maturity and pays interest.

Article 668 A loan contract shall be in written form, unless otherwise agreed between natural persons.

The contents of a loan contract generally include terms such as loan type, currency, purpose, amount, interest rate, term and repayment method.

Article 669 When concluding a loan contract, the borrower shall, at the request of the lender, provide the true information about the business activities and financial status related to the loan.