C21orf99 is located at 21q11.2.

C21orf99 has been reported to be expressed in normal testis, prostate, brain, placenta and trachea, and in brain, breast, colorectal, esophageal, gastric, gynecological, head and neck, lung, melanocytic, and prostate cancers at the mRNA level. C21orf99 has been detected in normal testis and in prostate cancers at the protein level.

Spontaneous humoral, but not cellular immune responses against C21orf99 have been detected in cancer patients.

The function of C21orf99 is unknown.