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