NOTCH4 and breast cancer: Among all four NOTCH receptors, NOTCH4 is more attractive in TNBC for the following reasons: (1) it was initially identified in virus triggered mouse mammary tumor (initially designated as int-3) and thus directly linked to tumor initiation 22; (2) NOTCH4 has been reported to be implicated in cancer progression 23 and BCSC regulation 24, and function as a melanoma stem cell marker 25; (3) NOTCH4 has been recently shown to play a role in TNBC, as two independent groups observed high expression of NOTCH4 in TNBC 26, 27 to highlight its clinical significance.