HTR5A and breast cancer: Given that growth inhibition by 5-HT occurs through the 5-HT7 receptor, resistance to growth inhibition in breast cancer cells could be explained by: 1] a loss of 5-HT7 receptor expression (Figure 4A and 4Bvi), 2] expression of 5-HT receptors that counteract 5-HT7 (that is, Gi-coupled), (Figure 4A, Bii and 6Biii) or 3] changes in signaling downstream of 5-HT7; and these possibilities are not mutually exclusive.