To construct the subject DNA useful for e.g. creating proteins bearing new function, through such processes that a gene coding a protein is segmented every unit domain, the resultant domains are linked to one another in any order followed by mixing so as to duplicate the ends of the respective unit domain DNAs in such an order as to intend them to be substituted, and an amplification is conducted using a primer.

A gene DNA coding a protein is segmented and isolated every unit domain such as into a DNA coding module, a DNA coding secondary structural portion, a DNA coding supersecondary structure, a DNA coding the original sequence and an exon domain DNA, and the isolated unit domain DNAs are linked to one another in any combination. The linked unit domain DNAs are then mixed so as to duplicate the ends of the respective unit domain DNAs in such an order as to intend them to be substituted, and an amplification is then conducted using a primer for the unit domain DNAs on both ends with the resultant mixture as template, thus affording the objective substituted variant gene DNA useful for e.g. creating proteins each having new function owing to the combination of such unit domains as exon and module.