Inhibits the activity of serine/threonine-protein phosphatases flw/PP1beta9C and Pp1-87B. Required for germ line cell viability and differentiation, normal transcriptional activity and maintenance of DNA integrity. Interacts with serine/threonine-protein phosphatases flw/PP1beta9C and Pp1-87B with higher affinity for Pp1-87B. In primary spermatocytes and maturing spermatids, found throughout the cytoplasm with a distinctive concentration in speckles. In the embryo, primarily cytoplasmic with some protein in speckles in both the nucleus and cytoplasm. In salivary glands, predominantly cytoplasmic with a mild perinuclear accumulation. Also detected on salivary gland polytene chromosomes where it is associated with regions of active transcription. Most abundant in early embryo, pupa and adult gonads (at protein level). In testis, expressed in mitotically proliferating spermatogonia and early primary spermatocytes with levels decreasing as spermatocytes mature and no expression in postmitotic stages. Embryos hatch normally but first instar larvae show very little locomotion or feeding and die soon after hatching. Embryos show transcriptional defects with reduced expression of ebony and Ass. Somatically-rescued mutant larvae show defective germ line cell viability and differentiation with damaged DNA. Belongs to the RNA polymerase II subunit 5-mediating protein family.