Regulates the timing of initiation of DNA replication (PubMed:29746464, PubMed:30277458, PubMed:26022086). Functions in copy number control by promoting the underreplication of DNA, which is found in many late replicating euchromatic regions of salivary gland polytene chromosomes (PubMed:30277458). Promotes underreplication by localizing to active DNA replication forks in a partially SuUR-dependent manner, and inhibiting replication fork progression (PubMed:30277458). Might also work as an adapter to recruit Pp1-87B to multiple sites on the chromosome and may function with Pp1-87B to mediate underreplication (PubMed:26022086, PubMed:29746464, PubMed:30277458). Plays an essential role in embryonic development, in the transition from larvae to pupae and, probably, in proliferating tissues later on (PubMed:26022086, PubMed:29746464). In embryos, during mid-blastula transition, binds to and selectively delays the replication of large blocks of repetitive DNA satellite sequences during S phase in response to the activity of Cdk1; maternal Rif1 is specifically required for the normal extension of S phase 14 (PubMed:29746464). Unlike mammalian orthologs, does not appear to play a role in DNA damage repair (PubMed:22712556, PubMed:26022086). Interacts with Pp1-87b (PubMed:26022086, PubMed:29746464). Interacts with SuUR (via SNF2-like region) (PubMed:30277458). Localizes to heterochromatin in all stages of amplifying follicle cells and accumulates at active DNA replication forks in the later stages of DNA amplification (PubMed:30277458). Associates with polytene chromosomes in salivary glands (PubMed:26022086). Associates with heterochromatin in a dynamic manner in different mitotic stages of the cell cycle (PubMed:22712556, PubMed:26022086, PubMed:29746464, PubMed:30277458). Absent at metaphase chromosomes but re-associates with the condensed chromosomes during late anaphase (PubMed:22712556, PubMed:26022086, PubMed:29746464). During mid-blastula transition in the S phase of embryos, dissociates from chromatin before the replication of the bound DNA satellite sequences (PubMed:29746464). Expressed in nurse cells and follicle cells in the adult female (at protein level) (PubMed:26022086, PubMed:30277458). Detected in adult at extremely low levels (PubMed:26022086). Expressed in embryos at high levels between 0-2 hours; from 8 to 14 hours, the highest levels are in the brain then diminishes to negligible levels (at protein level) (PubMed:22712556, PubMed:26022086, PubMed:29746464). Expressed in larval imaginal disks and salivary glands (at protein level) (PubMed:26022086). Detected in larvae at lower levels in imaginal disks, the central nervous system and ovary and, at extremely low levels, in testis (PubMed:26022086). Phosphorylated, probably by Cdk1; phosphorylation regulates dissociation from heterochromatin. Embryos have a reduced hatch rate and develop into adults with reduced survival (PubMed:29746464). During mid-blastula transition in embryos, S phase 14 is significantly shorter (PubMed:29746464). In larval salivary glands, loss of DNA underreplication resulting in abnormally enlarged chromocenters (PubMed:30277458). RNAi-mediated knockdown results in lethality (PubMed:26022086). Escaper flies only live for a few days and are infertile (PubMed:26022086). Females show increased necrotic spots in the abdominal segments (PubMed:26022086). RNAi-mediated knockdown in the imaginal disks does not affect viability (PubMed:26022086). Belongs to the RIF1 family. Highly divergent.