K5/6 was increased in serous ovarian cancer tissues following chemotherapy compared with chemonaïve tissues (P < 0.0001, Table 1B). Increased K5/6 immunostaining is evident in the images in Figure 2G and 2H that are an example of matching tissues from the same patient before and after chemotherapy treatment, respectively. Similar results were observed with the K5 monoclonal antibody (Table 1C, Supplementary Figure 1G and 1H). These findings are supported by mRNA expression studies in chemosensitive and chemoresistant primary ovarian cancer cells. KRT5 but not KRT6C mRNA expression was increased in primary cells derived from patients’ ascites with recurrent chemoresistant disease (n = 10) compared with primary cells from patients who responded to the chemotherapy treatment (n = 9; Figure 5A and 5B, P = 0.0006, Mann Whitney U test). KRT5 (Figure 5A), KRT6C mRNA (Figure 5B) and K5 protein (Figure 5C and 5D) were increased in OVCAR-5 cells made resistant to carboplatin (OVCAR-5-CBPR) compared with the parental OVCAR-5 cells by immunofluoresence. Furthermore, treatment with an IC50 dose of carboplatin significantly increased the number of K5+ cells in 4 serous ovarian cancer cell lines (OVCAR-5, OVCAR-3, OAW28 and COV362) (Figure 5C and 5D).