FGFR4 and rhabdomyosarcoma: Figure 2a shows the distribution of FGFR4 mutations in these studies, with the most frequently mutated sites being the FGFR4 hotspot mutations previously identified in rhabdomyosarcomas (N535 and V550).13–15 Although FGFR4 hotspot mutations are rarely detected in primary tumors (<0.05%), they are present in ~0.5–1% of breast metastases, significantly enriched relative to nonbreast metastases (~0.02%) (Fig. 2b).