Topic Areas

With the identification of multiple genes and gene families that exhibit a testis–tumor expression pattern, a number of key questions have emerged. The most active areas of research are listed below. These concepts were originally outlined in Lloyd Old’s 2007 review, “Cancer is a Somatic Cell Pregnancy.” Many groups are now studying these questions, yet much remains to be learned:

  1. What are the mechanisms that promote the abnormal expression of cancer–testis antigens (CTAs) in cancer?
    Extensive evidence suggests that DNA methylation is a key mechanism silencing CTA expression in somatic cells. However, the precise CpG sites, mechanisms of demethylation, and regulatory factors governing reactivation remain largely unclear.

  2. Why is CTA expression heterogeneous?
    A persistent and confounding observation is the high degree of inter- and intra-tumoral heterogeneity in CTA expression. The underlying reasons and mechanisms driving this variability are not yet understood.

  3. What are the functions of these proteins in the developing gamete and embryo?
    Knockout models in mice have shown that several CTAs are essential for gametogenesis, with loss resulting in male and, in some cases, female infertility. However, many human CTAs lack clear murine orthologues, complicating functional studies. In addition, several CTA families (e.g., GAGE, MAGE) have undergone significant expansion in humans, whereas only one or two family members exist in mice. The evolutionary basis and developmental significance of this expansion remain unknown.

  4. What is the function of CTAs in cancer cells?
    Over the past two decades, numerous studies have revealed that individual CTAs contribute to diverse cancer cell–autonomous and non-autonomous processes. Many CTAs enable cellular behaviors that promote resistance to cell death, continued proliferation, and enhanced survival. (See the Publications tab for recent studies.)

  5. Are there specific oncogenic mutations or molecular alterations that correlate with CTA expression and dependency?
    Many studies are investigating how CTA dependency arises in the context of the molecular drivers of tumorigenesis and whether specific oncogenic pathways promote CTA activation or reliance.

  6. Can CTAs be targeted therapeutically in patients?
    The discovery of CTAs was originally motivated by their potential as tumor-specific immunotherapy targets. Considerable progress has shown that CTAs such as NY-ESO-1 and PRAME, when recognized by T cells, can elicit potent anti-tumor responses. The repertoire of potential targets is extensive, but determining which CTAs represent viable T-cell antigens remains an active area of investigation. Moreover, for CTAs that are essential for tumor survival, small-molecule inhibition may represent an alternative therapeutic approach.

  7. What genes qualify as CTAs or cancer–germline (CG) genes, and how are they defined?
    Early discoveries suggested the existence of genes whose expression was absolutely restricted to testis and cancers. In 2005, Lloyd Old and colleagues established a reference set of ~270 genes that were highly enriched in testis and activated in tumors, now curated in the CTDatabase. This foundational work also revealed that many genes exhibit a testis-biased pattern with extremely low expression in a few somatic tissues, termed CT-Like Genes. His work also demonstrated that many CT-genes are found on the X chromosome. In 2025, Charles De Smet and colleagues refined this concept using single-cell, normal, and tumor expression datasets to define the most comprehensive and stringent set of testis/tumor genes to date. Their work both expanded and contracted the original list and introduced CTExplorer, an R/Bioconductor package for mining expression data across datasets.

    Defining CT-Genes: intent and interpretation
    In the end, what qualifies as a CT-gene depends on the intent of the inquiry. The strictest criterion, that expression is confined to germ cells and tumors, is essential when considering these proteins as immunotherapeutic targets. Yet there is significant nuance in how classification should be approached.

    • First, mRNA expression does not always correlate with protein accumulation. Defining protein expression in normal, tumor and testis tissue is critical.

    • Second, for immune targeting, additional factors such as gene copy number, antigen processing, and major histocompatibility complex (MHC) presentation influence the therapeutic index.

    • Third, genes with highly restricted expression in the testis and certain somatic cell sub-populations may uncover novel aspects of normal physiology that were previously unrecognized.

    • Fourth, expression of genes otherwise highly enriched in testis and tumors, but with sparse expression in somatic tissues may reflect the co-option of developmental/specialized molecular programs activated by cancer cells to drive pro-tumorigenic processes such as genomic instability, stress tolerance, immune evasion, and other hallmarks of cancer.

    Thus, while strict germline restriction remains a central and necessary definition for some studies, a more flexible framework can reveal new biology in normal cells and how tumors exploit germline and highly specialized molecular programs for survival and adaptation.