Adenocarcinomas exhibit great heterogeneity, with many genetic and epigenetic alterations. The Kelch domain-containing protein 7B (KLHDC7B) has recently been identified as epigenetically modified and upregulated in breast cancer. The potential reversibility of epigenetic states offers exciting possibilities for novel cancer diagnostics and drugs. However, to properly evaluate specific inhibitors, the role of KLHDC7B in the development and progression of breast cancer should be established. With that objective in mind, the present study investigated a series of human breast tumours and correlated their clinicopathology, according to the Elston-Ellis modification of the Scarff-Bloom-Richardson (SBR) grading system, with KLHDC7B mRNA expression, analysed using quantitative PCR (qPCR). The results revealed that KLHDC7B was significantly upregulated in grade 3 tumours, and that KLHDC7B expression varied according to the tumour grade and the individual, being downregulated in well-differentiated and moderately-differentiated tumours (grade 1–2) and upregulated in poorly-differentiated tumours (grade 3). Immunohistochemical staining revealed that ductal tumours and tumours with a higher percentage of Ki67 positive cells showed the highest levels of KLHDC7B. Receptor expression, HER, p53 status, presence of metastasis, and vascular invasion showed no association with KLHDC7B expression. Previous studies have proposed KLHDC7B as an epigenetic marker of breast cancer. We propose that KLHDC7B should be used as a marker for poorly-differentiated tumours only; use of KLHDC7B without considering tumour grade could lead to an inaccurate diagnosis. Finally, we suggest the appropriate breast cancer cell lines to use to determine the functions of KLHDC7B. KLHDC7B expression was tested in the non-tumour cell line MCF-10A and in the breast cancer cell lines MCF-7, MDA-MB-231 and MDA-MB-468, using qPCR and western blotting. The results revealed that all tested cancer cell lines overexpressed KLHDC7B mRNA, but MDA-MB-468 exhibited a much lower level of protein expression relative to mRNA. Although the breast cancer cell lines used may be appropriate for studying KLHDC7B epigenetic status, MDA-MB-468 should be excluded from functional experiments.