X-linked genetic disorders almost exclusively affect males. All female children will be unaffected, though some may be carriers. Parents who wish to avoid X-linked disorders may screen for sex and only implant the female embryos. This option costs much less than a custom test for PGD.
Immune system compatability
It is possible to use PGD to identify an embryo that is a match to a relative who needs a kidney or blood marrow transplant.
Say a couple already has one child, and that child has a life-threatening condition that can only be cured with a bone marrow transplant. But no compatible donor is available. If the couple has another child, it is possible that through chance alone the child will be a match. But by using PGD to screen for specific combinations of immune genes, the couple can greatly increase the likelihood that the child will be a match.
Some forms of deafness are inherited through variations in single genes. Some deaf parents hope to have a child who is also deaf. They consider deaf culture to be an important part of their lives, and they wish to have a child who can share that with them.
With the genetic basis of deafness becoming better understood, it is technically possible for deaf parents to use PGD to increase their chances of having a deaf child.