['Women diagnosed with pregnancy-associated breast cancer postpartum have a worse prognosis, stage for stage, than other women with breast cancer. The time of breast involution is tumor promotional. The extracellular matrix protein tenascin-C is upregulated during involution in animal models and promotes breast cancer progression. It interacts with transforming growth factor (TGF)β, which also is involved in breast involution and breast cancer progression. Little is known about the expression of tenascin-C during human breast involution, nor its relationship to TGFβ. The purpose of this study was to investigate the expression of tenascin-C throughout lactation, as well as its relationship to TGFβ1 and TGFβ2.', 'Three milk samples from 25 lactating women (transitional, whole, and wean) were collected, separated into components (cells, fat, and skim), and the skim fraction analyzed for total protein, tenascin-C, TGFβ1, and TGFβ2. Tenascin-C, TGFβ1, and TGFβ2 were detectable in all milk samples.', 'Highest tenascin-C levels on average were found in whole milk, whereas highest mean TGFβ1 and TGFβ2 levels were in wean milk. Wean samples on average had higher levels of both TGFβ1 (26%) and TGFβ2 (>500%) than matched transitional milk samples. Tenascin-C levels in wean milk were associated with nursing length (p = 0.048). Combining all three milk collection time points, tenascin-C exhibited a weak inverse correlation with TGFβ1 and TGFβ2 (p < 0.1). The inverse correlation at the wean time point was stronger for TGFβ2 than -1 (-0.37 versus -0.25). Tenascin-C, a protein known to promote breast cancer progression, is expressed throughout lactation.'] Does expression of the Extracellular Matrix Protein Tenascin-C vary During Lactation?