We have studied three unrelated families with X-linked recessive MF4 and as a result have: (1) identified three novel FGF16 mutations that help establish this as the cause for X-linked recessive MF4; (2) shown the capacity for variable phenotype of MF4 among family members with mutations in FGF16; and (3) found possible correlation with heart disease. This evidence concerns the gene FGF16 and syndactyly type 8.