Transcriptional activator that binds the sequence 5'-ATCAATCAA-3'. Activates transcription of PF4 in complex with MEIS1 (By similarity). Forms heterodimers with MEIS1 and heterotrimers with MEIS1 and HOXA9. Interacts with PBXIP1 (By similarity). Belongs to the TALE/PBX homeobox family.