ABCB6 and cancer: Another MRD characteristic that is better recapitulated in 3D microtumors is the increased expression of ATP‐binding cassette (ABC) transporters and markers for tumor‐initiating cells/cancer stem cells (TICs/CSCs): consistent with the fact that MRD lesions survive chemotherapy and are the source of ovarian cancer recurrences, we previously identified an ABC/TIC/CSC gene signature which we now find overexpressed in MRD 3D microtumors compared to MRD 2D (Figure 4B).