Wilms' tumor is a type of kidney cancer that mainly affects children. In most cases, Wilms' tumor affects only one of the two kidneys.

Kidneys
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In the fetus, some of the cells that are supposed to grow into mature kidneys stay as fetal kidney cells. Clusters of these cells sometimes remain after birth. If they do not mature within three or four years, they may grow without control or order. A large mass of these cells may develop into a Wilms' tumor.

A small percentage of children with Wilms' tumors have inherited an abnormal gene from one parent. This gene increases the chance that kidney cells will turn into a Wilms' tumor. Some children have certain birth defects that increase their risk for Wilms' tumor. However, many children with Wilms' tumors do not have any known inherited gene changes or birth defects. Researchers do not know why these children have some kidney cells that do not mature properly.