The phrase
"different isoforms" refers to slightly different versions of the same protein or DNA molecule that exist within a cell or organism. These isoforms may have variations in their structure or function, leading to different roles or effects within the body.
Full definition
Thus, although there are only three isoforms expressed in human skeletal muscle, there are many more hybrid muscle fiber types comprising muscle fibers with
several different isoforms within the same muscle fiber (see review by Scott et al. 2001).
For example, one isoform is highly expressed in skeletal muscles and
a different isoform is more specific to heart, brain or liver.