Natural History of Sphingosine Phosphate Lyase Insufficiency Syndrome (SPLIS)

All identified patients with SPLIS diagnosed by genetic criteria are eligible for enrollment in this study, regardless of baseline demographic, biochemical or metabolic features and regardless of interventions such as vitamin B6 supplementation, dialysis or kidney transplantation at time of enrollment. This study may include siblings of index SPLIS cases if the sibling has been genetically confirmed to have SPLIS, regardless of whether they have active disease at the time of enrollment. Data from deceased SPLIS patients will also be collected. Inclusion Criteria: Potential subjects fulfilling the following criteria will be eligible to participate in this study: 1. Living or deceased patients diagnosed with SPLIS based on 1. harbor biallelic pathogenic variant (PV) or likely PV (LPV) in the SGPL1 gene, regardless of phenotype OR 2. harbor nucleotide changes in both SGPL1 alleles, regardless of variant classification, if they also have one of the following: b1) exhibit at least 1 phenotypic feature of SPLIS (nephrosis, endocrine defect, ichthyosis, neuropathy, male gonadal dysgenesis, lymphopenia) b2) have evidence from biochemical or molecular data (such as enzyme expression or activity in skin fibroblasts) that indicate a possible loss of function in the S1P lyase (SPL) protein b3) are a sibling of a subject with nucleotide changes in both alleles of SGPL1 and at least 1 phenotypic feature of SPLIS 2. Informed consent and (if appropriate) assent for living subjects. For deceased subjects, the Principal Investigator (PI) will be responsible for ensuring that all requirements have been met in regard to the relevant local laws and regulations. Parents of participating SPLIS patients may be included as controls. Exclusion Criteria: Subjects with SPLIS (or their parents) who are currently using or have a history of using an investigational agent in the last 30 days with the exception of off-label use of medications will be excluded from the study