Authors have not cited articles that are absolutely relevant to this paper Malathy Shekhar, Wayne State University 19 February 2009 I read the article, "Novel multicellular organotypic models of normal and malignant breast: tools for dissecting the role of the microenvironment in breast cancer progression" by Holliday et al with great interest. However, I was disappointed to note that while the authors included references for homotypic three-dimensional cultures, citations pertinent to their paper were omitted. Specifically, original articles describing heterotypic interactions between breast cancer cells and stromal cells (the microenvironment), and the dominant role the microenvironment plays in breast tumorigenesis were omitted. Although citing these papers would indicate that the authors of this paper are not the first to report on multicellular interactions in three-dimensional tricultures, inclusion of these papers in their article would provide a better discussion of their findings. I have included the references to these papers. Breast stroma plays a dominant regulatory role in breast epithelial growth and differentiation: implications for tumor development and progression. Shekhar MP, Werdell J, Santner SJ, Pauley RJ, Tait L.; Cancer Res. 2001, Feb 15;61(4):1320-6. Interaction with endothelial cells is a prerequisite for branching ductal-alveolar morphogenesis and hyperplasia of preneoplastic human breast epithelial cells: regulation by estrogen. Shekhar MP, Werdell J, Tait L. Cancer Res. 2000 Jan 15;60(2):439-49. Competing interests No competing interests.