When a genetic disease (e.g., cystic fibrosis) is caused by many different mutations, with different frequencies among populations, there is no simple screening test that can detect all the mutations. Although 70% of patients with cystic fibrosis have a 3-base pair deletion that can be readily detected by PCR (the AF508 mutation), the remaining 30% have disease caused by several hundred allelic forms of CFTR. To detect all would require sequencing of the CFTR genes. This prohibits mass screening. The other listed options do not apply.