Genes may help determine why some mice (and perhaps people) become obese when eating a sugar- and fat-laden diet, while others can, to a certain extent, fend off the extra pounds. Working with about 100 types of mice, Aldons Lusis of UCLA and colleagues found that most strains gained weight when fed a high-fat, high-sucrose diet, but stopped gaining weight after a few weeks on the high-calorie diet. Only a few strains continued to pack on body fat over the course of the eight-week experiment. The finding could be evidence of a biological set point for body mass, the researchers report in the Jan. 8 Cell Metabolism, with some genetic variants disrupting the mechanism that creates the weight ceiling. The amount of body fat the mice packed on could be traced to 11 different genetic variants, including some implicated in human obesity.