DKK1 and bone metastasis: In BCa clinical samples, RSPO2, LGR4, and DKK1 expression was significantly increased in bone metastasis patient samples compared with patient samples with primary tumor only, and there was a positive correlation in bone metastasis clinical samples between LGR4 and DKK1 expression, between RSPO2 and DKK1, and between RANKL and DKK1.