CTDP1 is a phosphatase that serves critical roles in transcriptional regulation and cell cycle control. Mechanistically, CTDP1 processively dephosphorylates serine-2 and serine-5 residues in the heptad repeats of RNA polymerase II's C-terminal domain (CTD), promoting transcriptional elongation and reinitiation 1. The protein also regulates mitotic exit by dephosphorylating substrates including USP44, CDC20, and WEE1, which are essential for M-phase-promoting factor inactivation. Beyond transcription, CTDP1 participates in DNA damage responses through BRCA1 C-terminal domain-specific interactions with Fanconi anemia pathway proteins, particularly FANCI, enhancing homologous recombination repair and interstrand crosslink resolution 2. Biallelic Ctdp1 deletion causes embryonic lethality before day 7.5, while heterozygous mice are haplosufficient 3. Disease relevance is exemplified by Congenital Cataracts, Facial Dysmorphism, and Neuropathy (CCFDN) syndrome, an autosomal recessive disorder caused by a deep intronic CTDP1 founder variant predominantly affecting Vlax Roma populations 4. CCFDN presents with early-onset peripheral neuropathy, congenital cataracts, microcorneae, and developmental delays 5. Clinically, CTDP1 expression correlates with breast cancer survival and proliferation, positioning it as a potential therapeutic target 2. Emerging therapeutic approaches for CCFDN include antisense oligonucleotides and genome editing strategies 4.