NOVELTY - A thermostable AviIII peptide (I) comprising a fully defined sequence (S1) of 957 amino acids, as given in the specification, is new.    USE - The isolated polypeptide molecule (IV) is useful for reducing cellulose in a starting material, which involves administering (IV) to the starting material. The method further involves administering a second polypeptide molecule chosen from glycoside hydrolase family of proteins. The starting material is agricultural biomass or municipal solid waste. A set of amplification primers (VIII) is useful in an assay method for detecting a polynucleotide encoding the thermostable AviIII, comprising amplifying a nucleic acid sequence with (VIII), and correlating the amplified nucleic acid sequence with detected polynucleotide encoding the thermostable AviIII (all claimed). A thermostable AviIII peptide (I) is useful for degrading treated biomass into simpler forms of carbohydrate, which is used in the formation of ethanol or other industrial chemicals. (I) is also useful for treating fabrics to remove cellulose-containing stains.    ADVANTAGE - (I) can be used under extreme conditions, for example, at elevated temperatures and acidic pH.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a composition (C1) comprising a substantially purified thermostable AviIII peptide, the AviIII peptide comprising a catalytic domain glycoside hydrolase (GH)74 and a carbohydrate binding domain (CBD) III;    (2) an industrial mixture (II) suitable for degrading cellulose, comprising the thermostable AviIII polypeptide of (C1);    (3) an isolated polynucleotide molecule (III) encoding a thermostable AviIII polypeptide, the AviIII polypeptide comprising (S1), a catalytic domain sequence (S2), a CBD type III sequence (S3), or a sequence having 70 % sequence identity to (S1), (S2) or (S3);    (4) an isolated polypeptide molecule (IV) comprising (S1), (S2), (S3) or a sequence of (S2) and (S3), or a sequence having 70 % sequence identity to the above sequences;    (5) a fusion protein (V) comprising (IV) and a heterologous peptide;    (6) a cellulase substrate complex comprising (IV) bound to cellulose;    (7) a vector comprising (III);    (8) a host cell (VI) genetically engineered to express (IV);    (9) a composition comprising (IV), and a carrier;    (10) an isolated antibody (VII) that specifically binds to (IV);    (11) producing (M1) (IV), comprising incubating a host cell genetically engineered to express (IV);    (12) a set of amplification primers (VIII) for amplification of a polynucleotide molecule encoding a thermostable AviIII, comprising two or more sequences with nine or more contiguous nucleic acids derived from (III);    (13) a probe for hybridizing to a polynucleotide encoding AviIII, comprising a sequence of nine or more contiguous nucleic acids derived from (III);    (14) assessing the carbohydrate degradation activity of AviIII, comprising analyzing a carbohydrate degradation in the presence of AviIII and a carbohydrate degradation in the absence of AviIII on a substrate, and comparing the carbohydrate degradation in the presence of AviIII with the carbohydrate degradation in the absence of AviIII; and    (15) assessing (M2) the carbohydrate degradation activity of AviIII in the presence of an agent of interest, comprising analyzing a carbohydrate degradation in the presence of AviIII and a carbohydrate degradation in the presence of AviIII and the agent of interest on a substrate exposed, and comparing the carbohydrate degradation in the AviIII treated substrate with the carbohydrate degradation in the AviIII treated substrate in the presence of the agent of interest.