These genes are from GENCODE v27 (maybe they are not valid for any other annotation).
For more information about these Functional Isoforms, read the following publication:
An analysis of tissue-specific alternative splicing at the protein level
Jose Manuel Rodriguez, Fernando Pozo, Tomas di Domenico, Jesus Vazquez, Michael L. Tress
DOI:10.1371/journal.pcbi.1008287