Now a days it is possible to detect the mutated gene causing cancer by allowing radioactive probe to hybridise its complimentary DNA in a clone of cells, followed by its detection using autoradiography because :

Detection of a specific gene using a radioactive DNA probe is based on nucleic‑acid hybridisation.

The probe is a short DNA fragment labelled with a radioactive isotope. When mixed with DNA from a clone of cells, it will bind only to DNA strands that have a sequence exactly complementary to the probe.

If the gene is normal (wild‑type), the probe finds a perfectly matched sequence, forms a double‑stranded hybrid, and the radioactivity can be visualised on photographic film by autoradiography.

In a mutated cancer gene, one or more nucleotides in the target sequence are altered. Because the probe was designed for the normal sequence, the mutated region no longer provides the required base‑pair complementarity.

Consequently, the probe cannot hybridise to the mutated DNA. No radioactive duplex is formed, so the film receives no exposure at that location.

Therefore, the mutated gene does not appear on a photographic film because the probe has no complementarity with it.