RSPO1-4 genetic mutations are infrequent in primary and metastatic prostate cancer (0–0.6% incidence, Tables S1–S4 [64,65,66]), however, RSPO2 gene fusions that increase RSPO2 expression have previously been identified in 1.3% of mCRPC patients [67], and RSPO2 genetic variants are common in prostate cancer, especially in metastatic disease (Table 1). Here, RSPO1 is linked to prostate cancer.