IBSP and cystic fibrosis: Additionally, the difference in concentration patterns between IL-17A and IL-17F is unexpected because both cytokines are structurally similar and regulated similarly.15,25 However, IL-17F is associated with increased expression of the mature bone markers Col1, Col2, BSP, and osteocalcin.34 Nevertheless, the paradoxical decrease in IL-17F relative to IL-17A in CF warrants further investigation.