Cort Johnson in his blog Health rising asks if the fatigue found in MS and ME is similar or different:

Simply looking at the symptoms indicated that MS is primarily a fatiguing disorder, and that MS patients may, in fact, experience more fatigue than ME/CFS patients.  They do experience some PEM, but nothing like what shows up in ME/CFS.

“Fatigue is obviously present in Chronic Fatigue Syndrome, but it’s more of
a post-exertional malaise disorder. This study suggests Post-Exertional Relapse Syndrome would have been a better name for ME/CFS than Chronic
Fatigue Syndrome.

But what do the genes say?

Would these differences show up in the gene expression results?

They would..

The PEM Genes – Where ME/CFS and MS Part Ways

The expression levels of two genes (P2X4/TRPVI) that bounced up immediately
after exercise in the ME/CFS group, and then stayed elevated for 48 hours
were associated with post-exertional malaise. One of them, P2X4, was
directly associated with both the increased fatigue and pain experienced
after exercise in the ME/CFS patients. These are muscle metabolite sensing
genes that assess the levels of factors associated with muscle fatigue and
damage.

After exercise the MS patients looked more like healthy controls than the
ME/CFS patients. The levels of the metabolite sensing genes actually dropped
in both the MS and healthy controls eight hours after exercise and then
rebounded to normal levels. The Lights called this response evidence of a
“well-regulated sensory pathway” .

That sensory pathway looks more than a little dysregulated in ME/CFS.

Read more: A “Fatigue” Disorder No More? – What Multiple Sclerosis Taught Us
About Fatigue and Chronic Fatigue Syndrome