MSTN and breast cancer: Thus, the poor prognosis associated with HER2 overexpression in breast cancer (and its relationship to enhanced clinical response to chemotherapy) may also be related to the overexpression of Gasermin B. Titin-Cap (the TCAP gene product) blocks the secretion of myostatin, a transforming growth factor β (TGF-β) family member that negatively regulates cell growth (Nicholas et al. 2002).