CCN1 and plasma cell myeloma: However, our finding of intron 1-4 retention in myeloma cells, and the presence of an in-frame stop codon downstream to the CCN1 signal sequence (within intron 1) implies that intron 1-4 retaining CCN1 mRNA completely blocks endogenous translation of full length CCN1 protein in INA-6 cells, preventing translation of truncated variants that could be termed isoforms resulting from alternative splicing[15].