Component of the PAF1 complex (PAF1C) which has multiple functions during transcription by RNA polymerase II. PAF1C associates with RNA polymerase II, is involved in transcriptional elongation and in histone modifications including methylation on histone H3 'Lys-4' (H3K4me3) (By similarity). PAF1C seems to be required for multiple steps in cardiac formation. Involved in the migration of myocardial precursors to the midline and the differentiation of the atrioventricular boundary in the developing heart. Component of the PAF1 complex, which at least consists of cdc73, paf1, leo1, ctr9 and rtf1 (By similarity). The PAF1 complex interacts with PHF5A (By similarity). Interacts with ctr9 (PubMed:21338598). Belongs to the PAF1 family.