Cell proliferation requires calmodulin, a protein that regulates calcium-dependent enzymes involved in signal transduction pathways in eukaryotic cells. Calmodulin-like protein (CLP) is found in certain epithelial cell types, including normal breast epithelium, and, although it closely resembles calmodulin in amino acid sequence, CLP interacts with different proteins than does calmodulin. The observation that CLP mRNA expression is dramatically reduced in transformed breast epithelial cells led to two hypotheses: (1) CLP helps to maintain the differentiated state in epithelial cells; and (2) downregulation of CLP accompanies malignant transformation of breast epithelial cells. The objective of this study was to determine if the expression of CLP in human breast cancer specimens is reduced in comparison to its expression in normal breast tissue. Eighty human breast cancer biopsy specimens were analyzed immunohistochemically for CLP expression by using a polyclonal rabbit antihuman CLP antibody. CLP expression was reduced in 79% to 88% of the invasive ductal carcinoma and lobular carcinoma specimens and in a similar fraction of the ductal carcinoma in-situ specimens, compared with normal breast specimens. None of the breast cancer specimens showed an increase in CLP expression. These findings support the hypotheses that CLP behaves as a functional tumor suppressor protein and is downregulated early in breast cancer progression.