Human health and medicine

Lestaurtinib

Lestaurtinib (formerly CEP-701) is an oral, multi-targeted tyrosine kinase inhibitor of the indolocarbazole class that inhibits FLT3, JAK2, and TRKA, developed as a treatment for FLT3-mutated acute myeloid leukemia.

Letermovir

Letermovir (Prevymis, AIC246, MK-8228) is a CMV DNA terminase complex inhibitor, targeting pUL56, used as prophylaxis in CMV-seropositive allogeneic HSCT recipients and high-risk kidney transplant recipients; approved in the EU in 2018 and the US in 2017.

Lethal ataxia with deafness and optic atrophy

Lethal ataxia with deafness and optic atrophy, or Arts syndrome (OMIM 301835), is an extremely rare X-linked recessive PRPS1 loss-of-function disorder causing congenital deafness, ataxia, hypotonia, intellectual disability, and optic atrophy, usually fatal before age 5.

Lethal congenital contracture syndrome type 1

Lethal congenital contracture syndrome type 1 (LCCS1, Herva disease) is a Finnish autosomal recessive arthrogryposis caused by GLE1 mutations, causing fetal akinesia, hydrops, and prenatal death before 32 weeks.

Lethal congenital contracture syndrome type 2

LCCS2, also called multiple contracture syndrome Israeli-Bedouin type, is a rare autosomal recessive arthrogryposis caused by ERBB3 mutations on 12q13, marked by congenital contractures, neurogenic megacystis, and neonatal lethality.

Lethal congenital contracture syndrome type 3

Lethal congenital contracture syndrome type 3 (LCCS3, OMIM 611369) is an ultra-rare autosomal recessive arthrogryposis caused by PIP5K1C loss-of-function variants, like LCCS2 but without neurogenic bladder, lethal perinatally from respiratory insufficiency.

Lethal dose

Lethal dose (LD50, median lethal dose): the single dose killing 50 percent of test animals, introduced by J. W. Trevan in 1927; basis of GHS acute toxicity classes and replaced by OECD tests 420, 423, and 425.

Lethal multiple pterygium syndrome

Lethal multiple pterygium syndrome (LMPS, OMIM 253290) is a rare autosomal recessive fetal disorder with multiple skin webs, severe arthrogryposis, hydrops, and death before or shortly after birth; CHRNG is the main gene.

Lethal omphalocele-cleft palate syndrome

Lethal omphalocele-cleft palate syndrome (OMIM 258320, ORPHA:2736) is an autosomal recessive multiple congenital anomaly disorder combining omphalocele and cleft palate, lethal in infancy, with fewer than a handful of reported cases worldwide.

Letrozole

Letrozole (Femara, CGS 20267) is a nonsteroidal third-generation aromatase inhibitor approved in 1997 for hormone receptor-positive breast cancer in postmenopausal women, dosed 2.5 mg daily, also used off-label for ovulation induction.

Sep 29, 2026

Letter number sequencing

Letter number sequencing is a neuropsychological test task assessing working memory, in which the examinee recalls numbers in ascending order followed by letters in alphabetical order.

Letterer–Siwe disease

Letterer–Siwe disease is the obsolete eponym for aggressive multisystem Langerhans cell histiocytosis of infants, now classified as multisystem LCH with risk-organ (liver, spleen, bone marrow) involvement, historically fatal in over 50% of children under 2.

Leukapheresis

Leukapheresis (leukocytapheresis) is an apheresis procedure that removes white blood cells from circulated blood and returns the rest. Used for hyperleukocytosis cytoreduction, stem cell and CAR T-cell collection; main risk is citrate-induced hypocalcemia.

Leukemia

Leukemia is a group of cancers of the blood-forming tissues in which abnormal white blood cells crowd out normal marrow production, divided into acute and chronic myeloid and lymphoid types.

Leukemia (journal)

Leukemia is a monthly peer-reviewed medical journal covering leukemia and allied hematologic malignancies, founded in 1987 and published by Springer Nature since 2000, with a 2024 impact factor of 13.4.

Leukemia & lymphoma

Leukemia & Lymphoma (Leuk Lymphoma) is a monthly peer-reviewed journal on hematological malignancies founded in 1989, published by Taylor & Francis, MEDLINE-indexed since 1992, hybrid open access, 14 issues yearly.

Leukocyte Adherence Inhibition Test

Leukocyte Adherence Inhibition Test (LAI test), an in vitro assay of cell-mediated antitumor immunity first described by Halliday in 1972, in which cancer patients' leukocytes lose glass adherence when exposed to matching tumor extracts, mediated by LAIF and blocked by serum.

Leukocyte adhesion deficiency

Leukocyte adhesion deficiency (LAD): rare autosomal recessive immunodeficiency in which neutrophils cannot adhere to vessel walls and exit blood; subtypes LAD-I (ITGB2/CD18), LAD-II (SLC35C1), LAD-III (FERMT3); recurrent infections, neutrophilia, delayed cord separation.

Leukocyte Count

Leukocyte count, also called WBC count or white blood cell count, is a CBC blood test measuring leukocytes, normally about 4,500–11,000 cells/µL in adults; high counts (leukocytosis) suggest infection or inflammation, low counts (leukopenia) suggest marrow or immune disease.

Leukocyte Reduction Procedures

Leukocyte reduction (leukoreduction, leukofiltration) removes most donor white blood cells from red cell and platelet units via filtration to under 5.0 x 10^6 residual WBCs, preventing febrile non-hemolytic reactions, HLA alloimmunization, and CMV transmission.

Leukocyte Transfusion

Leukocyte (granulocyte) transfusion, GTX, is donor neutrophil transfusion for severe neutropenia with refractory bacterial or fungal infection; evidence from the RING trial and Cochrane reviews shows uncertain benefit, with dose-response signals and risks of pulmonary reactions and alloimmunization.

Leukocytoclastic vasculitis

Leukocytoclastic vasculitis (LCV), also called hypersensitivity vasculitis or cutaneous small-vessel vasculitis, is a histopathologic pattern of neutrophilic small-vessel vasculitis with leukocytoclasia and fibrinoid necrosis, presenting as palpable purpura of the lower legs.

Leukocytosis

Elevated white blood cell count (leukocytosis): WBC above 11,000/mm³ in nonpregnant adults, usually reactive from infection, inflammation, stress, smoking, or drugs, but sometimes leukemia; classified by cell type.

Leukodystrophy

Leukodystrophy: a group of over 50 rare inherited genetic disorders damaging CNS white matter myelin (also called the leukodystrophies). Progressive; diagnosed by MRI and genetic testing; treated with transplant or, for early MLD, Lenmeldy gene therapy.

Leukoencephalopathy with bilateral anterior temporal lobe cysts

Leukoencephalopathy with bilateral anterior temporal lobe cysts (ORPHA:139444) is a rare nonprogressive neurological disorder of infancy with intellectual deficit, spasticity, motor retardation, temporal lobe cysts on MRI, sensorineural deafness, and unknown, possibly autosomal recessive cause.

Leukoencephalopathy with brain stem and spinal cord involvement-high lactate syndrome

LBSL, leukoencephalopathy with brain stem and spinal cord involvement-high lactate syndrome (MIM 611105), is a rare autosomal recessive leukodystrophy caused by DARS2 variants, marked by selective tract MRI abnormalities, elevated white matter lactate, and progressive ataxia and spasticity.

Leukoencephalopathy with calcifications and cysts

Leukoencephalopathy with calcifications and cysts (LCC, Labrune syndrome) is a rare autosomal recessive SNORD118-related cerebral microangiopathy with the MRI triad of white matter disease, calcifications, and cysts, purely neurological and distinct from Coats plus.