TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Molecule: The codon adaptation index (CAI) of the nucleic acid fragment is 0.9-1.0, preferably 0.95-1.0. The cellobiohydrolase is a cellobiohydrolase II comprising an amino acid sequence selected from any one of fully defined 390-492 amino acid sequences (SEQ ID NO. 11-15) given in the specification. It is selected from C. heterostrophus C4 cel7, G. zeae K59 cel6, I. lacteus MC-2 cex3, V. volvacea cbhII-I, or Piromyces sp. E2 cel6A. The polynucleotide comprises a nucleic acid sequence selected from any one of fully defined 1190-1496 bp sequences (SEQ ID NO. 6-10) given in the specification, or a nucleic acid sequence that is at least 95% identical to SEQ ID NO. 6-10. The fragment of the cellobiohydrolase is a cellobiohydrolase signal peptide, where the signal peptide comprises an amino acid sequence selected from any one of fully defined 18-20 amino acid sequences (SEQ ID NO. 23, 25, 28, 31, or 34) given in the specification. The fragment of the cellobiohydrolase is a cellobiohydrolase CBM comprising an amino acid sequence selected from any one of fully defined 28-37 amino acid sequences (SEQ ID NO. 26, 29, 32, 35, or 36) given in the specification. The fragment of the cellobiohydrolase is a GH family 6 domain comprising an amino acid sequence selected from any one of fully defined 290-321 amino acid sequences (SEQ ID NO. 22, 24, 27, 30, or 33) given in the specification. The nucleic acid encodes 50-350 contiguous amino acids of the cellobiohydrolase. At least one nucleotide within a sequence of 4-10 consecutive A, T, C, or G nucleotides is replaced with a different nucleotide, where the nucleotide replacement does not alter the amino acid sequence encoded by the polynucleotide, and where the nucleotide replacement creates a codon that is the second most frequently used codon to encode an amino acid in Saccharomyces cerevisiae. At least one restriction enzyme site within the polynucleotide is removed by replacing at least one nucleotide within the restriction enzyme site with a different nucleotide, where the nucleotide replacement does not alter the amino acid sequence encoded by the polynucleotide, and where the nucleotide replacement creates a codon that is the second most frequently used codon to encode an amino acid in S. cerevisiae. Preferably, the restriction enzyme sites are selected from PacI, AscI, BamHI, BglII, EcoRI, or XhoI. One or more direct repeats, inverted repeats, and mirror repeats with lengths of 10 bases or longer within the polynucleotide is altered by replacing at least one nucleotide within the repeat with a different nucleotide, where the nucleotide replacement does not alter the amino acid sequence encoded by the polynucleotide and where the nucleotide replacement creates a codon that is the second most frequently used codon to encode an amino acid in S. cerevisiae. The polynucleotide is operably associated with a heterologous nucleic acid, which encodes a signal peptide, a secretion signal, or a carbohydrate binding module. The signal peptide is the S. cerevisiae alpha mating factor signal sequence. The carbohydrate binding module is the carbohydrate binding module of Trichoderma reesei Cbh1, T. reesei Cbh2, G. zeae Cbh2, I. lacteus Cbh2, V. volvacea Cbh2, or Piromyces sp. Cbh2. The polynucleotide and the heterologous nucleic acid encode a fusion protein, which is fused via a linker sequence. Preferred Vector: The vector further comprises an S. cerevisiae PGK1 promoter. It also comprises an S. cerevisiae PGK1 terminator. The vector further comprises the sequence of any one of fully defined 8174-8480 bp sequences (SEQ ID NO. 37-41) given in the specification. Preferred Host Cell: The host cell is a bacterial cell or a yeast cell. Specifically, the yeast is selected from S. cerevisiae, Saccharomyces pastorianus, Saccharomyces bayanus, Kluyveromyces lactis, Kluyveromyces marxianus, Schizosaccharomyces pombe, Candida albicans, Pichia pastoris, Pichia stipitis, Yarrowia lipolytica, Hansenula polymorpha, Phaffia rhodozyma, Candida utilis, Arxula adeninivorans, Debaryomyces hansenii, Debaryomyces polymorphus, Schizosaccharomyces pombe, or Schwanniomyces occidentalis. The host cell further comprises additional polynucleotides encoding heterologous cellulases selected from an endogluconase, a beta -glucosidase, or a cellobiohydrolase. The endogluconase, beta -glucosidase, or cellobiohydolase is selected from T. reesei endogluconase I, C. formosanus endoglucanase I, S. fibuligera beta -glucosidase I, T. emersonii cellobiohydrolase I, C. lucknowense cellobiohydrolase IIb, or T. reesei cellobiohydrolase II. The host cell comprises: (i) a heterologous polynucleotide encoding C. formosanus endoglucanase I; (ii) a heterologous polynucleotide encoding S. fibuligera beta -glucosidase I; (iii) a heterologous polynucleotide encoding T. emersonii cellobiohydrolase I; and (iv) a heterologous polynucleotide encoding C. heterostrophus C4 cel7, G. zeae K59 cel6, I. lacteus MC-2 cex3, V. volvacea cbhII-I, or Piromyces sp. E2 cel6A. The host cell can saccharify and/or ferment crystalline cellulose. The specific activity on Avicel is 0.08-0.20 mu mol/mg/minute. Preferably, the host cell produces ethanol. Preferred Method: In hydrolyzing a cellulosic substrate, the cellulosic substrate comprises a lignocellulosic biomass selected from grass, switch grass, cord grass, rye grass, reed canary grass, miscanthus, sugar-processing residues, sugarcane bagasse, agricultural wastes, rice straw, rice hulls, barley straw, corn cobs, cereal straw, wheat straw, canola straw, oat straw, oat hulls, corn fiber, stover, soybean stover, corn stover, forestry wastes, recycled wood pulp fiber, paper sludge, sawdust, hardwood, and/or softwood. EXAMPLE - No suitable example given.