Although expression of FSIP2 is testis-specific (Brown et al., 2003), we have found that FSIP2 has a higher mutation frequency not only in male reproductive system tumors, such as testicular germ cell tumors, but also in Paget disease, liver cancer and other cancers (Zhang et al., 2014, 2019; Litchfield et al., 2015). This evidence concerns the gene FSIP2 and Paget disease.