In some cancer, cells reproduce uncontrollably and form masses called tumors. what processes are not taking place that allow for this to occur

The processes that are not taking place which allow for tumors to form is that the cancer cells are not responding to the signals that regulate the growth of cells. Living cells use cell division to reproduce and to repair damages; cell division is tightly controlled by the body system. When cells stop responding to the regulating signals, tumors will be formed.