Since the 1990s, bacteria, fungi and yeasts have been genetically engineered for the industrial production of biofuels and bioproducts. More conventionally, the improvement of microorganisms for biomass conversion has been done using classical chemical mutagenesis, a random approach followed by the screening and selec­tion of a desired trait. Nevertheless, with advancements in molecular biology and biotechnology approaches, the improvement of microorganisms via rational engi­neering of proteins and metabolic engineering of path­ways has become more prevalent (Strohl, 2001). This is due to the economic needs of the industry, which demands the development of strains that produce greater yields and a different variety of products. Specifically, in the bioconversion of biomass, researchers face challenges related to the substrate such as appropriate enzymes for conversion and microorganisms that produce them, fermentation of nonglucose sugars (i. e. xylose), and "consolidated bioprocessing", where the production of enzymes for biomass conversion (i. e. cellulose produc­tion), hydrolysis or modification of the biomass (i. e. cellulose hydrolysis), and fermentation of solubilized carbohydrates occur in a single step (Lynd et al., 1999). Therefore, prior to engineering microorganisms for biomass conversion it is important to select host organ­isms with desired characteristics; with emphasis on strains that can utilize low-cost substrates, have high product yield, competitive fitness, and are more robust to environmental stresses (Lynd et al., 1999). Once a good host has been selected based on targeted physiolog­ical characteristics and functionalities, one can identify the additionally desirable characteristic that will then be engineered into the host, whether targeting proteins such as enzymes through rational engineering or chang­ing the metabolism and/or metabolic flux through meta­bolic engineering (Zhang et al., 2009).