To examine whether RSPO2/RANKL-DKK1 signaling is specific to bone metastasis patients, we next examined the serological RSPO2, RANKL, and DKK1 levels in benign breast lump patients (n = 9), primary BCa patients (n = 28), and bone metastasis BCa patients (n = 10). Here, DKK1 is linked to bone metastasis.