A 16-year-old girl presented with McCune-Albright syndrome associated with acromegaly and fibrous dysplasia. Brain MRI demonstrated a pituitary tumor. X-ray films showed bone deformities, and 99TmO4 bone scintigraphy revealed increased uptake of radioactivity in the affected bones. Although the serum FGF23 level was increased, the serum calcium, phosphate, and active vitamin D levels were all within normal limits. GNAS gene mutation was detected at neither codon 201 nor 227 by conventional PCR-based direct sequencing analysis. We performed a selective PCR with peptide nucleic acid (PNA) clamping to increase the sensitivity for gene mutation detection and identified the R201C GNAS mutation.