755 ILCS 5/4-9: Effect of alteration.

An addition to a will or an alteration, substitution, interlineation or deletion of any part of a will which does not constitute a revocation of a will is of no effect, unless made by the testator or by some person in his presence and by his direction and consent and unless the will is thereafter signed and attested in the manner prescribed by this Article for the execution of a will.