The domains of human PABPs. PABPC1, PABPC3, iPABP, PABPC5 and PABPN1 are shown, aligned on their first RNA-recognition motifs (RRMs). White capsules represent individual RRMs; black hexagons (5H) represent the five conserved helices at the carboxyl terminus. Inverted brackets indicate the site of expansion of a run of alanines in PABN1 that leads to the synthesis of PABPN1 with 12-17 alanines and results in the autosomal recessive disease oculopharyngeal muscular dystrophy (OMPD) [24, 25]. PABPN1 accumulates in OMPD patients and forms intranuclear inclusions that appear to sequester mRNAs and associated factors and promote cell death .