The DDX21 gene encodes a member of the DEAD box family of putative RNA helicases that are critical for various RNA metabolism processes, including ribosomal RNA biogenesis, RNA editing, RNA transport, and transcription regulation. Below is additional information on the DDX21 gene based on your requests:

### 1. Associated Genes:
While the gene summary primarily highlights DDX21's interaction with various RNA types, it may also interact with other genes involved in transcription and RNA processing. Some associated genes include:

- **SIRT7**: This gene interacts with DDX21 to prevent R-loop-associated DNA damage.
- **JUN**: DDX21 serves as a cofactor for transcription regulation by JUN, particularly influencing its phosphorylation.
- **TICAM1**: Part of a multi-helicase complex involving DDX21 that acts as a cytoplasmic sensor for viral RNA.
- **Various small nucleolar RNAs (snoRNAs)**: Such as SNORD56 and SNORD58, which are crucial in rRNA processing.

### 2. Aging and Age-related Diseases:
While detailed information is still emerging, DDX21's role in RNA metabolism implies potential connections to aging and age-related diseases, particularly in the context of:

- **Cellular Senescence**: Dysregulation of RNA processing mechanisms might impact cellular aging.
- **Neurodegenerative Diseases**: Altered RNA helicase function has been linked to conditions like Alzheimer's disease.
- **Cancer**: Abnormal expression or mutations in DDX21 may affect cellular growth and division, contributing to oncogenesis.

The specific association with age-related diseases is less defined but potentially relevant due to its fundamental roles in maintaining cellular RNA homeostasis.

### 3. Drug Interactions:
Currently, there are no specific drugs or drug classes known to directly target DDX21. However, compounds that modulate RNA helicase activity or influence RNA metabolism may indirectly affect DDX21 function. Drugs in antiviral therapy could also engage DDX21 in the context of its role in sensing viral RNA. Research in this area is still an evolving field.

### 4. Pathways and Biological Processes:
DDX21 is involved in several critical biological processes, including:

- **Ribosome Biogenesis**: Facilitates ribosomal RNA processing, assembly and modification.
- **RNA Processing**: Plays roles in splicing and modification of rRNA and snoRNA.
- **Transcription Regulation**: Modulates transcription by interacting with RNA polymerases I and II, particularly influencing the release of P-TEFb, which is critical for elongation during transcription.
- **Cellular Response to Viral Infection**: Acts as a sensor for double-stranded RNA as part of antiviral immune responses.
- **Deacetylation and DNA Damage Repair**: Works with SIRT7 to mitigate R-loop-mediated genomic instability.

### Conclusion:
DDX21 is an essential gene with a broad role in RNA metabolism, affecting vital processes such as transcription and ribosome biogenesis. Its involvement in aging and disease, as well as its potential interactions with other genes, presents significant avenues for further research, particularly in the context of cellular dysfunction and disease mechanisms.