STXBP4 and breast carcinoma: However, the cell-type-specific MiXcan results for STXBP4 suggest that stromal cells (estimated effect=0.18; p value = 2.1 × 10−4) may play a more important role than epithelial cells (estimated effect = –0.06; p value=0.41) in driving the positive association of tissue-level GReX with breast cancer risk.