In this Special Issue, we would like to focus on the various functions of the RAD52 helicase-like protein and the current implications of such findings for cancer treatment. Over the last few years, various laboratories have discovered particular activities of mammalian RAD52-both in S and M phase-that are distinct from the auxiliary role of yeast RAD52 in homologous recombination. At DNA double-strand breaks, RAD52 was demonstrated to spur alternative pathways to compensate for the loss of homologous recombination functions. At collapsed replication forks, RAD52 activates break-induced replication. In the M phase, RAD52 promotes the finalization of DNA replication. Its compensatory role in the resolution of DNA double-strand breaks has put RAD52 in the focus of synthetic lethal strategies, which is particularly relevant for cancer treatment.Over the last years, new functions of RAD52 have been identified. For example, various laboratories have discovered the involvement of mammalian RAD52 and RNA templates and RNA-DNA hybrid structures like R loops in unprecedented homology-directed DSB repair events [9]. First, RNA was discovered to serve as a bridging template in RAD52-and RPA-mediated homology-directed DSB repair, which may play a role during transcription, replication, class-switch recombination, and at telomeres. Second, in transcription-coupled HR (TC-HR) in G0/G1 cells, R loops generated during transcription were found to be bound by CSB, followed by RAD52-mediated, BRCA-independent RAD51 recruitment, and HR. Third, in transcription-activated HR (TA-HR) in S/G2 cells, RAD52 recruits the XPG cleaving R loops, which generates ssDNA overhangs for BRCA-dependent HR [9].Surprisingly, RAD52 also acts during events other than canonical DSB repair-namely, during DNA replication when it counteracts excessive fork regression, which may exhaust protection factors, causing breakage of reversed forks. At collapsed replication forks, RAD52 activates break-induced replication (BIR), a specialized pathway that repairs single-ended DSBs. During M phase, RAD52-mediated BIR also promotes the finalization of DNA replication (MiDAS-mitotic DNA synthesis). Notably, RAD52 cooperates with various nucleases, namely, MUS81 during BIR, ERCC1/XPF during SSA, and XPG during TA-HR. It also cooperates with MRE11 and MUS81/EME1 at de-protected forks that have reverted to process these structures into HR substrates, ultimately enabling the cell to continue DNA replication [10][11][12]. The multiple activities of RAD52 known to date are summarized in Figure 1.Cancers 2020, 12, x FOR PEER REVIEW 2 of 8