This paper is based on analytical work previously conducted by NewClimate Institute, Öko-Institut, Mitsubishi UFJ Research and Consulting Co., Ltd., and IGES. The transition of certified emission reductions (CERs) issued under the Clean Development Mechanism (CDM) for use by Parties towards their nationally determined contributions (NDCs) is a key outstanding issue for Article 6 negotiations at the UNFCCC. The latest draft presidency texts from December 2019 in Madrid include options for restricting which CERs may be used towards NDCs. To inform the ongoing negotiations, this paper presents updated estimates of the potential CER supply under different possible restrictions for the transition of CERs.