SMARCA4/BRG1 mutations account for 10%-15% of lung cancers. DNA fibers provide a microscopic foray into the realm of ongoing DNA replication events, demonstrating that loss of BRG1 promotes tumor progression not through over proliferation but via changes at replication forks due to increased firing of replication origins, contributing to fork stress and genome instability in lung cancers. For details, see article by Gupta and colleagues on page 3841.