Overview of 195 novel SEPs identified in K562 cells. (A) Length
of each SEP was determined using a defined set of criteria (see Methods), and the length distribution reveals that
the majority (>90%) of SEPs discovered are between 8 and 100 amino
acids. (B) SEPs utilize AUG, near cognate codons (i.e., one base away
from AUG), and unknown codons to initiate translations. (C) SEPs are
primarily derived from nonannoated RNAs (i.e., not found in RefSeq
database), but RefSeq RNAs do account for the production of 24% of
these SEPs. For the RefSeq-RNAs, the sORFs are found on coding RNAs
at the 3′-UTR and CDS and on noncoding RNAs such as antisense
RNAs and noncoding RNAs.