Most tumors are caused by an accumulation of genetic mutations. Knowing which cells these mutations originate in makes it easier to find treatments that target the cancer more surgically and avoid the extreme side effects of chemotherapy or radiotherapy. However, this is different with pediatric tumors, as the vast majority have very few underlying genetic alterations. This is the case with Ewing sarcoma, a rare type of cancer that occurs in the bones or the soft tissue surrounding the bones. It is cured in 60% of cases, but the toxicity of the treatment causes side effects in the children who receive it.Now, researchers from Hospital del Mar and the San Juan de Dios Research Institute have discovered that this aggressive tumor depends on a single genetic alteration originating in one cell, which they have also identified. "Until now, we thought that the oncogen—the gene that mutates and can produce cancer—needed more genetic alterations to produce the tumor, but now we have seen that this is not the case," explains Dr. Inmaculada Hernández-Muñoz, a researcher in the dermatological inflammatory and neoplastic diseases research group. Hernández-Muñoz details that they have managed to demonstrate that there is only one cell capable of tolerating this dysfunctional protein—the rest die or stop growing—and that a single oncogen is sufficient for the tumor to develop. The discovery, published in the journal Nature Communications, This opens the door to finding less aggressive therapies for Ewing sarcoma, especially considering that it is a tumor that affects children and that its side effects can affect them for life.