The oncogenic capabilities of the cell cycle protein cyclin D1 have long been established in a breast cancer setting. The CCND1 gene is amplified in up to 15 % of breast tumors, with overexpression of its corresponding protein found in up to 50 % of cases. While gene amplification is consistently associated with reduced patient survival times and treatment resistance, repeated attempts to clarify the prognostic and predictive impact of the cyclin D1 protein in breast cancer have yielded contrasting results. Here, we recommend that any examination of cyclin D1 in a patient cohort should begin by determining CCND1 copy number, with subsequent removal and separate analysis of amplified cases. Next, the remaining tumors should be examined for cyclin D1 protein expression in the context of well-defined breast cancer subgroups. Only in this manner can the true clinical value of cyclin D1 be fully elucidated.