### CDK3 Gene Overview

The CDK3 gene encodes a member of the cyclin-dependent kinase (CDK) family, specifically known for its role in regulating the cell cycle. The protein product of this gene is integral for transitioning cells from the G0 phase (quiescence) into the G1 phase, and subsequently from G1 to the S phase, where DNA synthesis occurs. It achieves this by interacting with cyclin C and phosphorylating key substrates that regulate the cell cycle.

### 1. Associated Genes

In addition to its direct interactions with cyclin C (CCNC) and retinoblastoma protein (RB1), CDK3 is associated with several other genes and proteins involved in cell cycle regulation and related processes, including:

- **E2F Family (E2F1, E2F2, E2F3)**: Transcription factors that regulate the expression of genes essential for cell cycle progression.
- **ATF1**: A transcription factor implicated in various cellular responses, which gets phosphorylated by CDK3.
- **CABLES1**: A protein involved in linking cell signaling and the regulation of the cell cycle.
- **Histone H1**: Phosphorylation of histone H1 by CDK3 is associated with chromatin remodeling during the cell cycle.

### 2. Aging and Age-Related Diseases

While the specific role of CDK3 in aging is not exhaustively characterized, there are notable associations with age-related processes:

- **Cellular Senescence**: Dysregulation of cell cycle proteins, including CDKs, is often seen in senescent cells. As cells enter senescence, their capacity to divide is diminished, potentially implicating CDK3 in the aging process.
- **Cancer**: CDK3's role in phosphorylating RB1 places it in a position of interest in cancer biology, as aberrant cell cycle regulation contributes to tumorigenesis. Increased activity of CDK3 may be related to certain cancers, particularly where G1/S transition control is impaired.

### 3. Drug Interactions

The interaction of CDK3 with small molecule drugs and drug classes has not been extensively studied compared to other CDKs, but there are broader classes of drugs affecting CDK activity which may indirectly impact CDK3:

- **CDK Inhibitors**: Compounds that target cyclin-dependent kinases in general are of interest, and these can potentially affect CDK3. Examples include:
  - **Palbociclib** (Ibrance): A selective inhibitor for CDK4/6, affecting cell cycle regulation.
  - **Ribociclib** (Kisqali) and **Abemaciclib** (Verzenio): Similar to Palbociclib, these are selective for CDK4 and CDK6 but highlight the therapeutic interest in targeting CDK pathways.

### 4. Pathways and Biological Processes

CDK3 is involved in multiple biological pathways and processes, including:

- **Cell Cycle Regulation**: Primarily during transitions from G0 to G1 and G1 to S phases.
- **Phosphorylation Signaling**: Involves mediating phosphorylation of proteins such as RB1, ATF1, and histone H1 that are crucial for chromatin structure and transcriptional activation.
- **E2F Activation Pathways**: Facilitates E2F1, E2F2, and E2F3 activation, contributing to the expression of genes necessary for DNA replication and cell cycle progression.
- **Cell Proliferation and Transformation**: Promotes processes that lead to cell division and growth, with implications in both normal physiology and oncogenesis.

### Summary

In summary, the CDK3 gene is a crucial player in the regulation of the eukaryotic cell cycle, having roles that span various biological processes and potential implications in aging and cancer. It interacts with several key proteins and is influenced by drug classes targeting cell cycle proteins, albeit specific information on direct interactions with drugs remains less well-defined compared to other CDKs.