NOVELTY - A polypeptide (D1) having a variant cellobiohydrolase I (CBH I) catalytic domain as compared to a reference CBH I catalytic domain, comprising specific amino acid substitution of Trichoderma reesei CBH I, is new.    USE - For saccharifying biomass, where the biomass is corn stover, bagasses, sorghum, giant reed, elephant grass, miscanthus, Japanese cedar, wheat straw, switchgrass, hardwood pulp, softwood pulp, crushed sugar cane, energy cane, or Napier grass; for producing ethanol (claimed); for the conversion of cellulose into fermentable sugars and for related fields of cellulosic material processing such as pulp and paper, textiles and animal feeds; in detergent compositions; for the treatment of mechanical pulp; for use as a feed additive and in grain wet milling; and used in variety of agricultural and industrial applications.    ADVANTAGE - The polypeptides retains at least 50% of the CBH I activity of a reference CBH I lacking the Arg268 substitution and/or Arg411 substitution; exhibit reduced product inhibition by cellobiose; and have improved productivity, thus maintain their reaction rates during the course of saccharification reaction. The IC50 of cellobiose towards a variant CBH I polypeptide is at least 2-fold the IC50 of a reference CBH I lacking the Arg268 substitution and/or Arg411 substitution.    DETAILED DESCRIPTION - A polypeptide (D1) having a variant cellobiohydrolase I (CBH I) catalytic domain as compared to a reference CBH I catalytic domain, comprising substitution at the amino acid position corresponding to Arg268 of Trichoderma reesei CBH I (Arg268 substitution); and/or a substitution at the amino acid position corresponding to Arg411 of Trichoderma reesei CBH I (Arg411 substitution), where the substitution decreases product inhibition as compared to the reference CBH I catalytic domain, is new. INDEPENDENT CLAIMS are included for the following:    (1) new polypeptide (D2) comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence corresponding to positions 18-444 of amino acid sequence containing 514 amino acids (SEQ ID NO: 2), as given in the specification, which has Arg-268-lys substitution and Arg-411-Ala substitution as compared to a protein of SEQ ID NO: 2;    (2) a composition (C1) comprising the polypeptide (D1) or (D2);    (3) a fermentation broth (C2) comprising the polypeptide (D1) or (D2);    (4) saccharifying biomass (M1), involving treating biomass with the composition (C1) or with the fermentation broth (C2);    (5) producing (M2) ethanol, involving (a) treating biomass with the composition (C1) or with the fermentation broth (C2), thereby producing monosaccharides, and (b) culturing a fermenting microorganism in the presence of the monosaccharides produced in step (a) under fermentation conditions, thereby producing ethanol;    (6) new nucleic acid (A1) comprising a nucleotide sequence encoding the polypeptide (D1) or (D2);    (7) a vector (V1) comprising the nucleic acid (A1);    (8) new recombinant cell (L1) engineered to express the nucleic acid (A1);    (9) new host cell (L2) transformed with the vector (V1);    (10) preparation of the polypeptide (D1) or (D2);    (11) generating (M3) a product tolerant variant CBH I polypeptide, involving modifying the nucleotide sequence of CBH I-encoding nucleic acid so that the nucleic acid encodes a variant CBH I polypeptide, where the variant CBH I polypeptide comprises Arg268 substitution and/or Arg411 substitution, and expressing the variant CBH I polypeptide, thereby generating a product tolerant variant CBH I polypeptide; and    (12) generating (M4) a nucleic acid that encodes a product tolerant variant CBH I polypeptide, involving modifying the nucleotide sequence of CBH I-encoding nucleic acid so that the nucleic acid encodes a variant CBH I polypeptide, where the variant CBH I polypeptide comprises Arg268 substitution and/or Arg411 substitution, thereby generating a nucleic acid that encodes a product tolerant variant CBH I polypeptide.