A protein having luciferase activity and at least 60% similarity to luciferase from Photinus pyralis, Luciola mingrelica, Luciola cruciata or Luciola lateralis, Hotaria paroula, Pyrophorus plagiophthalamus, Lampyris noctiluca, Pyrocoelia nayako or Photinus pennsylanvanica, wherein in the sequence of the enzyme, at least one of (a) the amino acid residue corresponding to residue 214 in Photinus pyralis luciferase (b) the amino acid residue corresponding to residue 232 in Photinus pyralis luciferase (c) the amino acid residue corresponding to residue 295 in Photinus pyralis luciferase (d) the amino acid residue corresponding to amino acid 14 of the Photinus pyralis luciferase (e) the amino acid residue corresponding to amino acid 35 of the Photinus pyralis luciferase (f) the amino acid residue corresponding to amino acid residue 105 of the Photinus pyralis luciferase (g) the amino acid residue corresponding to amino acid residue 234 of the Photinus pyralis luciferase (h) the amino acid residue corresponding to amino acid residue 420 of the Photinus pyralis luciferase (i) the amino acid residue corresponding to amino acid residue 310 of the Photinus pyralis luciferase is different to the amino acid which appears in the corresponding wild type sequence and wherein the luciferase enzyme has increased thermostability as compared to an enzyme having the amino acid of the corresponding wild-type luciferase at this position.
