As genes encoding cyclin-dependent kinases were cloned from fission yeast, budding yeast, and animal cells, investigators working in plant systems designed experiments to determine whether plants also possessed CDKs that functioned as the catalysts for cell cycle progression. To isolate a cDNA encoding a CDK in maize, investigators aligned the predicted amino acid sequences of human cdc2, S. pombe cdc2, and S. cerevisiae cdc28 genes (see the figure below). Degenerate oligonucleotide PCR primers corresponding to the boxed amino acids were generated and a DNA fragment of the expected size was synthesized by PCR using these primers and maize cDNA library as a template. Why were the boxed regions selected as the basis for primer design?