Pour cake mix into a large bowl. Add the egg and oil; mix until well blended. Mixture will be slightly dry. Reserve one cup for the topping, and pat the rest into an ungreased 9x13 inch pan.
Bake for 15 minutes in the preheated oven. Set aside to cool. Meanwhile, in a medium bowl, beat the cream cheese with the sugar and lemon juice until smooth. Spread the mixture evenly over the baked crust. Crumble the reserved cake mix mixture over the top.
Bake for an additional 15 minutes in the preheated oven, or until filling is set and the topping is lightly toasted. Cool before cutting into bars. Refrigerate leftovers.