The OTUD1 protein level decreased as the tumor spheres derived from SKOV3 or OVCAR8 cells underwent differentiation (Fig. 1c and Supplementary Fig. 1d and 1e); coincidentally, samples of high-grade serous ovarian cancer (HGSOC), which is a poorly differentiated subtype and possesses a higher mRNA expression-based stemness index (mRNAsi) (Supplementary Fig. 1f), contained higher levels of OTUD1 than samples of low-grade serous ovarian cancer subtypes (Fig. 1d). Here, OTUD1 is linked to ovarian serous adenocarcinoma.