Mono-leucine dependent basolateral sorting signal consisting of the amino acid sequence X₁h₂X₃h₄Lp₅p₆, wherein: X₁ represents a polar amino acid residue or alanine, h₂ represents any hydrophobic amino acid residue, X₃ represents any amino a residue, h₄ represents any hydrophobic amino acid residue, except leucine and isoleucine L represents a leucine residue, p₅ represents any polar amino acid residue, and p₆ represents any polar amino acid.