K5 is usually detected using a K5/6 combination monoclonal antibody (clones D5/16B4) as it is closely related to K6. Co-expression of K5 and K6 has been reported in a number of different types of neoplasms including basal cell carcinoma , prostate cancer [34, 35], ductal breast carcinoma [36–38], mesothelioma [39, 40], lung carcinomas , melanoma, basal cell carcinoma, and salivary gland tumors . K5/6 overexpression is associated with poor prognosis of basal-like breast cancers [17, 18, 42] and was found to be an independent indicator of recurrence-free survival and/or OS in breast cancer [17, 19, 20, 43]. The K5/6 monoclonal antibody has been used for the diagnosis of poorly differentiated squamous carcinomas and undifferentiated nasopharyngeal carcinomas [21, 22]. It can distinguish between small cell lung carcinomas which are K5 negative and malignant mesothelioma which are K5 positive [21, 22]. K5/6 has superior sensitivity and reliability in differentiating between benign and malignant prostate glands when compared with K903 (high molecular weight keratins); and  it has been used successfully in a five antibody panel (which also targets TRIM29, CEACAM5, SLC7A5, MUC1) to better classify the subtypes of lung carcinoma . The expression of K5/6 together with p63 has also been used to differentiate between adenosquamous carcinomas and adenocarcinomas in pleural effusion samples [45, 46]. Recently, K5 positive basal cells have also been identified as progenitors of bladder cancers .