CD6 and spondylolysis: STAM binding protein (STAMBP) and T‐cell surface glycoprotein CD6 isoform (CD6 isoform) can increase the risk of spondylolisthesis/spondylolysis, whereas monocyte chemoattractant protein 2 (MCP2) and latency‐associated peptide transforming growth factor beta 1 (LAP‐TGF‐β1) can decrease spondylolisthesis/spondylolysis risk.