(* Only Claim 1) Claim 1
A DNA sequence cassette encoding a dominant selectable marker effective in prokaryotic and eukaryotic transformants which comprises:

a) an ATG start codon in reading frame with the codons of a modified truncated aminoglycoside phosphotransferase-I (aph-I) gene, designated herein mtaph-I, and
b) at least one cassette-unique restriction site upstream of and proximal to the ATG start codon, said mtaph-I defined herein as the coding sequence for aph -I modified so as to retain the amino acid sequence encoded by the aph-I gene, but to eliminate any restriction site otherwise duplicated of said cassette-unique restriction site, and
truncated by deletion of codons encoding at least one amino acid in positions 2 - 10 inclusive.

A recombinant expression vector capable of conferring resistance to G418 on a prokaryotic or eukaryotic transformant which vector comprises:

• an ATG start codon in reading frame with the codons of the mtaph-I gene, all operably linked to appropriate control sequences functional in the host harboring the vector,
  said mtaph-I defined herein as the coding sequence for aph-I modified so as to retain the amino acid sequence encoded by the aph-I gene, but to eliminate any restriction site otherwise duplicated of a cassette-unique restriction site, and
  truncated by deletion of codons encoding at least one amino acid in positions 2 - 10 inclusive.

An expression vector, operable in eukaryotic host cells, which comprises:

• mtaph-I coding sequences operably linked to control sequences functional in the eucaryotic host,
  
  said mtaph-I defined herein as the coding sequence for aph-I modified so as to retain the amino acid sequence encoded by the aph-I gene, but to eliminate any restriction site otherwise duplicated of a cassette-unique restriction site, and
  truncated by deletion of codons encoding at least one amino acid in positions 2 - 10 inclusive.

An expression vector, operable in eucaryotic host cells, which comprises:

• a coding sequence for a fusion protein operably linked to control sequences functional in the eucaryotic host,
  wherein the encoded fusion protein comprises:
  a) a desired peptide as the N-terminal sequence and
  b) mtaph-I as the C-terminal sequence, said mtaph-I defined herein as the coding sequence for aph-I modified so as to retain the amino acid sequence encoded by the aph-I gene, but to eliminate any restriction site otherwise duplicated of a cassette-unique restriction site, and
  truncated by deletion of codons encoding at least one amino acid in positions 2 - 10 inclusive.

A fusion protein which comprises:

a) an N-terminal sequence comprising the N-terminal amino acid sequence of a desired protein and
b) a C-terminal sequence comprising mtaph-I, said mtaph-I defined herein as the coding sequence for aph-I modified so as to retain the amino acid sequence encoded by the aph-I gene, but to eliminate any restriction site otherwise duplicated of a cassette-unique restriction site, and
truncated by deletion of codons encoding at least one amino acid in positions 2 - 10 inclusive.

A fusion protein which comprises:

a) an N-terminal sequence comprising the N-terminal amino acid sequence of B-isopropylmalate dehydrogenase and
b) a C-terminal sequence comprising mtaph-I, said mtaph-I defined herein as the coding sequence for aph-I modified so as to retain the amino acid sequence encoded by the aph-I gene, but to eliminate any restriction site otherwise duplicated of a cassette-unique restriction site, and
truncated by deletion of codons encoding at least one amino acid in positions 2 - 10 inclusive.

A fusion protein which comprises:

a) an N-terminal sequence comprising the N-terminal amino acid sequence of yeast enolase and
b) a C-terminal sequence comprising mtaph-I, said mtaph-I defined herein as the coding sequence for aph-I modified so as to retain the amino acid sequence encoded by the aph-I gene, but to eliminate any restriction site otherwise duplicated of a cassette-unique restriction site, and truncated by deletion of codons encoding at least one amino acid in positions 2 - 10 inclusive.

A method of purifying a fusion protein which method comprises:

• subjecting a fusion protein containing mtaph-I to an amino glycoside containing support, said mtaph-I defined herein as the coding sequence for aph -I modified so as to retain the amino acid sequence encoded by the aph-I gene, but to eliminate any restriction site otherwise duplicated of said cassette-unique restriction site, and truncated by deletion of codons encoding at least one amino acid in positions 2 - 10 inclusive.