TOPBP1 and lymphopenia: The DDR pathway is crucial for lymphocyte development as demonstrated by the fact that deficiency or loss-of-function mutations in important DDR genes such as ATM, Nbs1, 53BP1, TopBP1, and DNA ligase IV cause multiple defects in lymphocyte development and function, resulting in aberrant V(D)J rearrangements, lymphopenia, and increased susceptibility to hematological malignancies (49–54).