Exon-specific knockdown of RUNX1 isoforms and effect on CD56 mRNA levels in K562 acute myeloid leukemia cells. Small inhibitory RNAs complementary to the RUNT-domain (affecting all 4 isoforms), exon 5a containing the transactivating domain 2 (affecting p48 and p30), the new exon 5.4 (affecting p30 and p24), and a nonsilencing control small inhibitory RNA (NS control) were used. Results confirm the expression of the different isoforms in the control cells and the appropriate down-regulation of each RUNX1 isoform with small inhibitory RNA. CD56 expression is down-regulated by inhibition of p48, but increased when only p30 and p24 are targeted. One representative experiment of 4.