A stepladder with four individually adjustable, self-locking rails so that the ladder can be placed on uneven terrain then safely adjusted to maintain level orientation. A method for preparing same, cells incorporating said DNA molecules and uses thereof in gene therapy are also described. Respective snap formations on the parts are fittable together only in the closed position in a condition of one of the parts bearing with prestress on the other part for securing the parts together against separation. The first V-shaped groove arms further vary in width as well as depth from a narrower vertex arm region to a wider medial arm region to a narrow terminal arm region so that the volume of the groove arms per length unit is substantially constant in the medial arm region.