CCDC66 and cancer: RNA transcripts, such as mRNAs, lncRNAs and circRNAs, are reported to serve as competitive endogenous RNAs (ceRNAs) in cancer regulation.3 Among them, circRNAs are highly stable and therefore have advantages as ceRNAs.4 And circRNAs are reported to play vital roles in cancer progression by functioning as miRNA sponges.5 In colon cancer, circRNA CCDC66 sponges suppressor miRNAs to induce cancer proliferation and metastasis.6 And circHIPK3 sponges miR‐124 to regulate cell growth.7 But the potential involvement of circRNAs in breast cancer metastasis to lung is not clear so far.