SFRP4 and skin basal cell carcinoma: Patched Homolog 1 (PTCH1) is the SHH receptor and is the most commonly mutated tumor suppressor in BCC, whereas adenomatous polyposis coli (APC), secreted frizzled-related protein 1 (SFRP1), SFRP2, SFRP4, and SFRP5 are all negative regulators of the canonical WNT pathway.