The invention relates to a new, more potent, coagulation Factor IX (FIX) expression cassette for gene therapy of Haemophilia B (HB). Disclosed is a vector for expressing factor IX protein, the vector comprising a promoter, a nucleotide sequence encoding for a functional factor IX protein, and an intron sequence, wherein the intron sequence is positioned between exon 1 and exon 2 of the nucleotide sequence encoding for a functional factor IX protein, and wherein the intron sequence has at least 80% identity to the sequence of SEQ ID NO. 1 as disclosed herein.
