any mutation in the native sequence shortle in science vol 229 pgs 1193 1201 1985 reviews techniques for mutating nucleic acids which are applicable to the present invention preferably mutants of the protein of the present invention are produced by site specific oligonucleotide directed mutagenesis e g zoller and smith methods in enzymology vol 100 pgs 468 500 1983 and mark et al u s pat no 4 518 584 entitled u201c human recombinant interleukin 2 muteins u201d which are incorporated by reference or by so called u201c cassette u201d mutagenesis described by wells et al in gene vol 34 pgs 315 323 1985 by estell et al in science vol 233 pgs 659 663 1986 and also essentially by mullenbach et al in j biol chem vol 261 pgs 719 722 1986 and by feretti et al in proc natl acad sci vol 83 pgs 597 603 1986 polypeptides with amino acid modifications i e muteins may be desirable in a variety of circumstances for example undesirable side effects might be reduced by certain muteins particularly if the side effect is associated with a different part of the polypeptide from that of the desired activity in some expression systems the native polypeptide may be susceptible to.