LSM10 is a U7-specific Sm-like protein that functions as a core component of the U7 snRNP complex, which processes replication-dependent histone pre-mRNAs 1. In the U7 snRNP, LSM10 replaces the spliceosomal SmD1 protein within a specialized seven-protein Sm ring 1, and its incorporation is dictated by the unique Sm-binding sequence of U7 snRNA 2. LSM10 localizes to Cajal bodies and histone locus bodies where it participates in the 3'-end cleavage of histone mRNAs, generating the characteristic stem-loop structure rather than poly(A) tails 3. Beyond 3'-end processing, LSM10 is essential for cell cycle regulation. Depletion of LSM10 causes cell cycle arrest in early G1 phase, preventing progression to S phase 4. Additionally, LSM10-containing U7 snRNP represses histone gene transcription during cell cycle arrest through interaction with hnRNP UL1, restricting histone synthesis to S phase 5. While LSM10 overexpression increases U7 snRNA levels, it does not enhance histone pre-mRNA processing activity, indicating that other factors like ZFP100 are limiting for catalytic function 4. The LSM10/LSM11-containing U7 snRNP composition is evolutionarily conserved from invertebrates to mammals 6, highlighting its fundamental importance in controlling histone gene expression during DNA replication.