a, Subcellular fractionation of 18S, Uph and Malat1 lncRNA in mouse neonatal cardiomyocytes (n = 3 biological replicates from 1 of 5 independent experiments; mean ± s.e.m.). b, In vitro transcription and translation of a plasmid encoding the major Uph RNA. A plasmid encoding the myoregulin (MLN) micropeptide was used as a positive control, and myoregulin with a frameshift mutation (MLN RNA-FS) was used as a negative control. In contrast to myoregulin, Uph and the negative control (MLN RNA-FS) did not produce any detectable proteins, indicating that Uph is a bona fide lncRNA. For gel source data, see Supplementary Fig. 1.