Abstract: A mutation in the human sonic hedgehog gene is associated with tumorigenesis. A variety of human tumors, including basal cell carcinomas, breast carcinomas, medulloblastomas, etc., have a somatic mutation that results in an amino acid substitution at position 133 ?*his133 SHH!, or in a mutation at position 114. Such mutated genes and fragments thereof, encoded protein, and antibodies specific for the mutated protein are useful in characterizing the phenotype of associated tumors. The mutant protein is useful in drug screening for compositions that antagonize or otherwise modulate HH activity or expression. The encoded protein is also used as a therapeutic, to modulate cell proliferation and differentiation, and treatment of pathological conditions associated with decreased hedgehog signaling.