What is the difference between a paralog and an isoform?
I think the confusing thing here is that the two terms come from different fields of biology. While describing something similar, they emphasise different aspects.
The term paralog is mostly used in evolutionary biology, when one wants to find out how genes developed. Paralogs are, as you correctly found out, genes that originate from the same ancestral gene which duplicated. Thus, you would use this word when you want to emphasise the evolutionary relationship.
The term isoform on the other hand is not interested in evolutionary origin, and in fact it is not perfectly defined. This word is used when people studying the molecular biology of an organism discover that there are proteins in this organism that have a very similar amino acid sequence. Here, the interesting question is what differences in function these similar, but slightly different proteins might have. The evolutionary origin is not asked for, and thus there are two different definitions of isoforms:

Some people define isoforms only as variants of the same gene that underwent different post-transcriptional modifications (e.g. English Wikipedia), thus, paralogs would not be isoforms.

Other people also allow paralogs in addition (e.g. German Wikipedia).


In your paper, I guess that the author wants to emphasise that the proteins are not the results of different splicing but encoded by two different genes.