The phrase "are merely a subset" is correct and usable in written English.
It can be used when discussing a group or category that is part of a larger group, often to clarify relationships between different elements.
Example: "The mammals in this region are merely a subset of the larger class of vertebrates."
Alternatives: "are simply a part" or "are just a portion".
Some say "entertainment" but I believe that is merely a subset of escape, as an experience.
It is useful to recall that, in the context of the CAP theorem [40, 41], consistency is often seen as the premise that all nodes see the same data at the same time [42] (i.e., the CAP version of consistency is merely a subset of the ACID version of the same property [41]).
Nineteen species (35%) were collected only in the two wetter (and warm) months (December and March) and no species were unique to the dry season (June and September) (Table 1), which suggests that the flightless invertebrate community in winter is merely a subset of the summer wet season community.
The domestication begins at time Td in a small founder population of the cultivated species, which is merely a subset of the members in the wild progenitor.
Although the dataset extracted from the SILVA SSU Ref database used in the evaluation of SINA is of high quality, it is merely a subset of the SILVA SSU seed.
Groupon's growth has been nothing short of extraordinary, but it's merely a small subset of an even larger category which I'd like to call online-to-offline commerce, or On2Off (O2O) commerce, in the vein of other commerce terms like B2C, B2B, and C2C.
Specifically, we observe that more than half of human disease genes are of ancient, pre-animal origins (52%), a number significantly larger than would be expected if disease genes were merely a random subset of all human genes (42.7%, p = 1.2 × 10−25 per one-tailed hypergeometric test; see Additional files 2 and 3).
