Mutant form of a cytotoxic ribonucleolytic protein which allows production by recombinant methods

A recombinant Onc protein, comprising:

• a polypeptide of SEQ ID NO:1 having a glutamine residue at position 1, a hydrophobic residue other than methionine at position 23, a lysine at position 9, a histidine at position 10, a histidine at position 97, a lysine at position 31, a phenylalanine at position 98, and a threonine at position 35, and no more than a single conservative amino acid substitution at other than positions 1, 9, 10, 23, 31, 35 and 98, wherein the conservative amino acid substitution does not introduce a methionine into the polypeptide;
• and
• a methionine at position-1 that can be subsequently cleaved to allow cyclization of the glutamine residue at position 1 to form a pyroglutamyl residue, such that the resulting protein is ribonucleolytic and cytotoxic.