RecruitingClinical trialSomatic Mosaicism in Twins Discordant for Childhood CancerSomatic mosaicism in cancer associated genes is one potential explanation for discordance in childhood cancer that has not been fully explored to date. This pilot study will focus on twins with central nervous system (CNS) tumors who are identified through the Children's Oncology Group's Project: EveryChild (PEC) registry or volunteer.