K5/6 immunostaining was abundant in the skin epidermis (Figure 2A) but little or no staining was observed in the ovarian surface epithelium (OSE) of normal ovaries (8/8, Figure 2B, Table 1A). High K5/6 immunostaining (score 2 or 3) was present in 25% (2/8) of the benign serous cystadenoma (Figure 2C, Table 1A), 60% (6/10) high grade serous borderline tumors (Figure 2D, Table 1A) and 29.9% (35/117) of the serous ovarian cancer cases (Table 1A). Sixteen percent (19/117) of the serous ovarian cancer tissues were negative for K5/6. Examples of low (score = 1) and high (score = 3) K5/6 immunostaining in serous ovarian cancer tissues are shown in Figure 2E and 2F, respectively. K5/6 immunostaining was increased in serous borderline tumors and serous carcinomas compared to normal ovaries (P = 0.006, Chi-Square test, Table 1A). Similar staining patterns were observed with a monoclonal antibody which detects only K5 (Table 1C), (Supplementary Figure 1). However, a higher proportion of serous carcinomas (66%, 70/106) had high K5 immunostaining compared to K5/6 immunostaining. Neither K5/6 nor K5 immunostaining were associated with patient age, FIGO stage, tumor grade or the presence of residual disease (Supplementary Table 1).