The present invention relates to a nucleic acid molecule which encodes a polypeptide having cellulase activity and which is thermostable, wherein said nucleic acid molecule comprises or has a nucleotide sequence selected from: i) a nucleotide sequence which encodes an amino acid sequence as set forth in SEQ ID NO. 2 or as set forth in SEQ ID NO. 8; ii) a nucleotide sequence which encodes an amino acid sequence having at least 80% sequence identity to the amino acid sequence of (i); iii) a nucleotide sequence which encodes a part of the amino acid sequence of (i); iv) a nucleotide sequence which encodes a part of the amino acid sequence of (ii) wherein said part has at least 90% identity to an amino acid sequence of SEQ ID NO. 4, SEQ ID NO. 6, SEQ ID NO. 14, SEQ ID NO. 16, SEQ ID NO. 18 or SEQ ID NO. 20; v) a nucleotide sequence as set forth in SEQ ID NO. 1 or as set forth in SEQ ID NO. 7; vi) a nucleotide sequence having at least 80% sequence identity to the nucleotide sequence of v) or a nucleotide sequence which is degenerate thereto; vii) a nucleotide sequence which is a part of the nucleotide sequence of v); viii) a nucleotide sequence which is a part of the nucleotide sequence of vi) wherein said part has at least 90% identity to a nucleotide sequence of SEQ ID NO. 3, SEQ ID NO. 5, SEQ ID NO. 13, SEQ ID NO. 15, SEQ ID NO. 17 or SEQ ID NO. 19; or a nucleic acid which comprises a nucleotide sequence which is complementary to the nucleotide sequence of any one of i) to vii). The invention further relates to polypeptides encoded by such a nucleic acid molecule, to constructs, vectors and cells comprising the nucleic acid molecule and to a use and method of using a polypeptide of the invention for degrading cellulose.