PTH and Eiken syndrome: The delayed ossification seen in Eiken syndrome patients having the homozygous E35K or Y134S mutations located in the receptor’s ECD can be linked to weakened interactions with PTHrP that lead to an impaired desensitization response specifically to PTHrP and not PTH (ligand-dependent biased desensitization) and thus an extended cAMP signal in the context of continued PTHrP exposure.