SPP1 and breast cancer: Amongst the cDNAs expressed at a higher level in the R37-OPN cells than in the R37 cells were previously characterized breast cancer-associated genes such as TPTI [14], ARNT [15], CSFIR [16], MDM2 [17], CD44 [18], Cxcr4 [19], RAN GTPase [20], cytokeratin 20 (CK20) [21], Ki67 [22], with fold increases of 8, 8, 9, 9, 11, 23, 25, 32, and 45, respectively.