In humans, generally men are affected and women are carriers for two reasons. The first is the simple statistical fact that if the X-chromosomes is a population that carry a particular X-linked mutation at a frequency of 'f' (for example, 1%) then that will be the frequency that men are likely to express the mutation (since they have only one X), while women will express it at a frequency of f 2 (for example 1% * 1% = %) since they have two X's and hence two chances to get the normal allele. Thus, X-linked mutations tend to be rare in women. The second reason for female rarity is that women who express the mutation must have two X chromosomes that carry the trait and they necessarily got one from their father, who would have also expressed the trait because he only had one X chromosome in the first place. If the trait lowers the probability of fathering a child or induces the father to only have children with women who aren't carriers (so as not to create daughters who are carriers rather than expressers and then only if no genetic screening is used) then women become even less likely to express the trait.
The structure of cyclocreatine is fairly flat (planar), which aids in passive diffusion across membranes. It has been used with success in an animal study, where mice suffered from a SLC6A8 (creatine transporter at the blood brain barrier) deficiency, which is not responsive to standard creatine supplementation.  This study failed to report increases in creatine stores in the brain, but noted a reduction of mental retardation associated with increased cyclocreatine and phosphorylated cyclocreatine storages.  As demonstrated by this animal study and previous ones, cyclocreatine is bioactive after oral ingestion   and may merely be a creatine mimetic, able to phosphorylate ADP via the creatine kinase system. 
XLI can be suspected based on clinical findings, although symptoms can take varying amounts of time to become evident, from a few hours after birth, up to a year in milder cases. The diagnosis is usually made by a dermatologist , who also typically formulates the treatment plan (see below). STS enzyme deficiency is confirmed using a clinically available biochemical assay. Carrier detection can be performed in mothers of affected sons using this test (see Genetics, below).  Molecular testing for DNA deletions or mutations is also offered, and can be particularly useful in the evaluation of individuals with associated medical conditions (see below). Prenatal diagnosis is possible using either biochemical or molecular tests. However, the use of prenatal diagnosis for genetic conditions that are considered to be generally benign raises serious ethical considerations and requires detailed genetic counseling.