Method for the separation of cells

1. A cultured mammalian cell containing a gene encoding a conditionally expressed transgenic extracellular surface marker that is detectable on the surface of the cell and exhibits one or both of resistance to digestion with trypsin and non-internalization by the cell upon binding of an antibody, wherein the gene encoding a conditionally expressed transgenic surface marker comprises:

(i) a promoter for driving the transcription of a first transcription sequence, operably linked to

(ii) a said first transcription sequence for preventing transcription of a second transcription sequence, thereby avoiding an immune response against the cell, wherein the first transcription sequence is conditionally removable by a CRE recombinase or a FLP recombinase, and

(iii) a said second transcription sequence encoding the extracellular surface marker, comprising

(a) a transmembrane or membrane association domain that anchors the surface marker in the cell membrane wherein at least a portion of the surface marker is on the extracellular side of the cell membrane, and

(b) a tag sequence for rendering a cell sortable,

whereby the first transcription sequence is conditionally removable such that the second transcription sequence is transcribable, and

whereby the surface marker renders the cell sortable through the detection of the conditionally expressed transgenic surface marker on the extracellular surface of the cell.

2. The cell according to claim 1 , wherein the conditionally expressed transgenic extracellular surface marker is expressed ectopically as a modified form of an endogenously expressed protein, whereby the modified form of endogenously expressed protein does not fulfill the same function as the endogenously expressed protein.

3. The cell according to claim 1 , wherein the first transcription sequence is flanked on either side by a recombinase recognition site for conditional removal of the first transcription sequence from the gene, wherein the recombinase recognition sites flanking the first transcription sequence are the same.

4. The cell according to claim 1 , wherein the second transcription sequence is a gene from the group consisting of LNGFR, H2Kk, CD133, CD271, CD2, CD14, CD90, or CD45 in an unmodified or modified form.

5. The cell according to claim 1 , wherein the promoter is

a heterologous promoter,

a ubiquitous or tissue specific promoter, and/or

a constitutive or inducible promoter.

6. The cell according to claim 1 , wherein the promoter is a promoter of a gene from the group containing: actin, CAGGS, hCMV, PGK, FABP, Lck, CamKII, CD19, Keratin, Albumin, aP2, Insulin, MCK, MyHC, WAP, Col2A, Mx, tet, Trex, and ROSA26.

7. An isolated mammalian cell containing a gene encoding a conditionally expressed transgenic extracellular surface marker that is detectable on the surface of the cell and exhibits one or both of resistance to digestion with trypsin and non-internalization by the cell upon binding of an antibody, wherein the gene encoding a conditionally expressed transgenic surface marker comprises:

(i) a promoter for driving the transcription of a first transcription sequence, operably linked to

(ii) a said first transcription sequence for preventing transcription of a second transcription sequence, thereby avoiding an immune response against the cell, wherein the first transcription sequence is conditionally removable by a CRE recombinase or a FLP recombinase, and

(iii) a said second transcription sequence encoding the conditionally expressed extracellular surface marker,

comprising

(a) a transmembrane or membrane association domain that anchors the surface marker in the cell membrane wherein at least a portion of the surface marker is on the extracellular side of the cell membrane, and

(b) a tag sequence for rendering a cell sortable,

whereby the first transcription sequence is conditionally removable such that the second transcription sequence is transcribable, and

whereby the surface marker renders the cell sortable through the detection of the conditionally expressed transgenic surface marker on the surface of the cell.