Our knowledge of cell cycle regulatory mechanisms in apicomplexan parasites is very limited. In this study， we describe a novel factor that has a vital role in chromosome replication and the regulation of cytoplasmic and nuclear mitotic structures， and we named this factor ECR1 for essential for chromosome replication 1. ECR1 was discovered by complementation of a temperature-sensitive (ts) mutant that suffers lethal， uncontrolled chromosome replication at 40°C similar to a ts mutant carrying a defect in topoisomerase. ECR1 is a 52-kDa protein containing divergent RING and TRAF-Sina-like zinc binding domains that are dynamically expressed in the tachyzoite cell cycle. ECR1 first appears in the unique spindle compartment of the (centrocone) of the nuclear envelope in early S phase and then in the nucleus in late S phase where it reaches maximum expression. Following nuclear division， but before daughter parasites separate from the mother parasite， ECR1 is downregulated and is absent in new daughter parasites. The proteomics of ECR1 identified interactions with the ubiquitin-mediated protein degradation machinery and the minichromosome maintenance complex， and the loss of ECR1 led to increased stability of a key member of this complex， MCM2. ECR1 also forms a stable complex with the cyclin-dependent kinase (CDK)-related kinase， Crk5 (TgCrk5)， which displays a similar cell cycle expression and localization during tachyzoite replication. Importantly， the localization of ECR1/TgCrk5 in the centrocone indicates that this -specific spindle compartment houses important regulatory factors that control the parasite cell cycle. Parasites of the apicomplexan family are important causes of human disease， including malaria， toxoplasmosis， and cryptosporidiosis. Parasite growth is the underlying cause of pathogenesis， yet despite this importance， the molecular basis for parasite replication is poorly understood. Filling this knowledge gap cannot be accomplished by mining recent whole-genome sequencing data because apicomplexan cell cycles differ substantially and lack many of the key regulatory factors of well-studied yeast and mammalian cell division models. We have utilized forward genetics to discover essential factors that regulate cell division in these parasites using the model. An example of this approach is described here with the discovery of a putative E3 ligase/protein kinase mechanism involved in regulating chromosome replication and mitotic processes of asexual stage parasites.