The properties of these mutation clusters support a direct link between SBS5+SBS40 and TLS, consistent with observations of a decreased number of SBS5 mutations in breast cancer cell lines lacking the TLS polymerase REV1 [59] (see also [48]) and a positive correlation between TLS polymerase theta (Polθ) expression levels and SBS5 mutation burden across human cancers [46]. This evidence concerns the gene REV1 and breast cancer.