### USP21 Gene Overview

**Gene Symbol**: USP21  
**Enzyme Class**: Ubiquitin carboxy-terminal hydrolases (C19 peptidase family)

The USP21 gene encodes a deubiquitinating enzyme involved in regulating various cellular processes through the removal of ubiquitin and ubiquitin-like proteins from target proteins. 

### 1. Associated Genes

While the summaries provided focus primarily on the functions and specific interactions of USP21, it is also linked to or interacts with several other proteins and genes not explicitly mentioned in the provided summaries:

- **BAZ2A**: Deubiquitination of this gene product leads to its stabilization.
- **Ribosomal Proteins**: Specifically interacts with RPS10 (eS10) and RPS20 (uS10), affecting the ribosome quality control mechanism.
- **ZNF598**: The regulation of ZNF598-mediated 40S ribosomal protein ubiquitination is impacted by USP21.

### 2. Aging and Age-related Diseases

Although there is limited direct evidence, the function of USP21 in regulating protein stability and gene expression suggests possible associations with aging:

- **Cellular Aging**: The deubiquitination activity of USP21 could influence the aging process by regulating the degradation of proteins involved in cellular stress responses.
- **Neurodegenerative Disorders**: Perturbations in protein homeostasis and the ubiquitin-proteasome system have been implicated in neurodegenerative diseases, highlighting a potential role for USP21 in these conditions.
- **Cancer**: Since USP21 is involved in transcription regulation, its dysregulation could affect oncogenic pathways, making it of interest in studies of cancer as an age-related disease.

### 3. Drug Interactions

Currently, there is limited information on specific drugs or drug classes that interact directly with USP21. However, due to its role in Ubiquitin Proteasome System (UPS), drugs targeting ubiquitin proteasome pathways could indirectly affect USP21 function:

- **Proteasome Inhibitors**: Drugs like bortezomib could impact the balance of ubiquitination/deubiquitination within cells, thereby influencing USP21 activity or expression.
- Potential exploration of small molecules for modulating deubiquitination could also be relevant going forward.

### 4. Pathways and Biological Processes

USP21 is involved in several key biological processes and pathways:

- **Protein Homeostasis**: By controlling the ubiquitination state of target proteins, USP21 plays a crucial role in maintaining protein balance within the cell.
- **Histone Modification and Gene Expression**: It modulates chromatin structure through the deubiquitination of histones, directly impacting transcription initiation and epigenetic regulation.
- **Ribosome Quality Control (RQC)**: Regulates RQC by deubiquitinating ribosomal proteins, helping to maintain ribosomal integrity during stress conditions.
- **Cell Signaling Pathways**: Involved in various signaling cascades that influence cell growth, differentiation, and apoptosis.

### Conclusion

In summary, USP21 is an essential deubiquitinating enzyme with diverse biological roles, particularly in gene expression regulation, protein homeostasis, and quality control mechanisms. Its interactions with other proteins and implications in aging and disease make it a significant target for further research, especially concerning therapeutic interventions in age-related conditions and cancers.