There is provided a nucleic acid molecule comprising a nucleotide sequence encoding a Factor VIII protein, wherein a B domain portion of the Factor VIII protein is encoded by a nucleotide sequence between 90 and 111 nucleotides in length and has an amino acid sequence that is at least 85% identical to SEQ ID NO: 4 which comprises six asparagine residues. Also provided is a Factor VIII protein, a vector comprising the above nucleic acid molecule, a host cell, a transgenic animal, a method of treating Haemophilia Haemophilia A, and a method for the preparation of a parvoviral gene delivery vector.