We successfully manufactured >12 000 vials of secondary BCR-ABL1 lyophilized cell reference panel, using the same K562 and HL-60 cell lines and following similar manufacturing procedures as the primary WHO panel (Supplementary Information).14 An MR4.5 level was added to the secondary panel, to enable more accurate IS calibration at this critical level, as CML patients reaching this deep molecular response are increasingly being considered for treatment cessation. Here, ABL1 is linked to chronic myelogenous leukemia, BCR-ABL1 positive.