Complete deletion of both the short arm of chromosome 1 (1p) alongside the deletion of the long arm of chromosome 19 (19q) (1p/19q co-deletion) is the molecular genetic signature of oligodendrogliomas, a subtype of primary brain tumours accounting for approximately ten to fifteen percent of all diffuse gliomas in adults. 1p/19q co-deletion is a pathognomonic biomarker that defines a distinct glioma entity and is characteristic of oligodendrogliomas. Approximately 70%, 50%, and 50% of oligodendrogliomas have deletions of 19q, 1p, and of both 19q and 1p, respectively. The codeletion of chromosome 1p and 19q is an early genetic event in oligodendroglial tumorigenesis. Oligodendrogliomas are diffusely infiltrating gliomas representing the second most common primary parenchymal brain tumor after astrocytomas/glioblastomas. The ISH study of the 1p/19q deletion aids in diagnosing oligodendroglioma tumors and predicting the response of an oligodendroglioma to therapy.