In amending claims, certain claim format rules must be followed. If these rules are not followed when the amended claims are presented in an amendment, the Examiner will so notify the applicant in a letter. The applicant must respond to the letter within one month of the mailing date of the letter or within the period of response set in the last Office action, whichever is later.
The position of the USPTO on this issue is described in the following section(s) of the Manual of Patent Examining Procedure (MPEP):
MPEP 608.01(m) - Form of Claims
Law - Compliance with Claim Format Rules