1. A polypeptide characterized in that said polypeptide is fragment of human GAX (Growth-Arrest-Specific Homeobox) protein comprising at least residues and having transcription repressor activity and/or exerting positive or negative influence on DNA replication. 2. The polypeptide according to claim 1 characterized in that it further comprises residues 140-230 of human GAX protein. 3. A polypeptide characterized in that said polypeptide contains at least one fragment, selected from fragments 1-32, 33-302 and 1-104 of human GAX protein and having transcription repressor activity and/or positively or negatively affects DNA replication. 4. A polypeptide characterized in that said polypeptide contains at least one fragment, selected from fragments 1-32, of human GAX protein and reactable with Ki protein. 5. A polypeptide according to one of claims 1-3, characterized in that said polypeptide is able to react with PCNA. 6. A polypeptide according to one of claims 1-3, characterized in that said polypeptide is able to react with Ki and/or PCNA and to form at least two-parted or, at least, three-parted complex with these proteins. 7. A polypeptide characterized in that said polypeptide in addition to a fragment of the GAX protein having transcription repressor activity, it further comprises at least a fragment of a different origin. 8. A polypeptide according to claim 7, characterized in that a fragment of a different origin has a biological activity and is marker or a element for defining a target. 9. Nucleic acid, encoding a polypeptide according to one of claims 1, 2, 5, or 6. 10. Nucleic acid, encoding a polypeptide according to one of claims 3, 5, and 6 11. Nucleic acid, encoding a polypeptide according to one of claims 4-6. 12. Nucleic acid, encoding a polypeptide according to claim 7. 13. Nucleic acid, according to one of claims 9-12 characterized in that said nucleic acid is cDNA 14. Expression cassette comprising nucleic acid according to one of claims 9-13 under control of promoter. 15. Expression vector comprising nucleic acid according to one of claims 9-13 or expression cassette according to claim 14. 16. Vector according to claim 15, characterized in that it is a virus vector. 17. Vector according to claim 16, characterized in that it is adenovirus. 18. Vector according to claim 16, characterized in that it is retrovirus. 19. Use of polypeptide according to one of claims 1-7 for repressing the gene transcription and/or positively or negatively affecting replication. 20. Use of polypeptide according to one of claims 1-7 for obtaining two- or three-parted complex with Ki or PCNA proteins and making negative or positive influence on cellular cycle. 21. Pharmaceutical composition, characterized in that it contains at least one polypeptide according to one of claims 1-7. 22. Pharmaceutical composition, characterized in that it contains vector according to on eof claims 15-18. 23. Method of screening and/or determining of polypeptides, interacting with GAX protein or one of GAX domains characterized in that the polypeptide is used according to one of claims 1-7. 24. Method according to claim 23 characterized in that the polypeptide is selected from fragments 1-32 and 33-302 of GAX protein.