The invention relates to a DNA sequence comprising a nucleotide sequence encoding a prepro-neuropeptide Y (preproNPY) where the leucine amino acid in position 7 of the signal peptide part of said preproNPY has been replaced by proline. The invention concerns further the mutant signal peptide or the mutant signal peptide associated with any other cleavage product of preproNPY, methods for the determination, in a biological sample, of the DNA sequence or the peptide. Furthermore, this invention relates to a method for diagnosing a predisposition for increased serum cholesterol or LDL cholesterol level in a human subject, and to methods for treating a human subject diagnosed for predisposition for increased serum cholesterol or LDL cholesterol. The present invention also relates to transgenic animals carrying either the mutant sequence or the normal sequence.