A donor's blood would only be a match for Zainab if they, too, are missing the Indian B antigen, according to OneBlood.
OneBlood says it has found three matches so far, one near London and two in the US, but she will need blood transfusions for the foreseeable future, which means more donors must be found.
OneBlood has orchestrated the search for Zainab as she will need numerous blood transfusions in the future.
Around 800 children in the United States are diagnosed each year with the disease and most are diagnosed when they are younger than five.
Donors need to be exclusively of Pakistani, Indian or Iranian descent, meaning both parents must be 100 percent of one of these ethnicities. "This was the worst thing we were expecting".
While the blood won't cure Zainab's cancer, it will allow her to undergo two bone marrow transplants, which will make her stronger and allow doctors to give her higher doses of chemotherapy, Forbes said.
The problem is that Zainab has a rare blood type.
Locating people who are missing the Indian B antigen comes down to genetics.
"The possibility of us finding a compatible donor for this little girl within the right ethnic group we want to screen is less than four percent", says Frieda Bright, OneBlood's Reference Laboratory Manager.
Even within that population, only 4% of people have the unusual genetic variation, according to OneBlood, the Florida-based charity that is leading the search.
OneBlood said it's working closely with other blood centers as well as the American Rare Donor Program, which helps search the globe for donors with rare blood types. The donors who belong to the rare blood group should coordinate with OneBlood to ensure the other additional compatibility testing is performer before donation.
But of those, only three donors have her rare blood type - including one in London. The girl needs to be completely supported by blood donations so she can survive the treatment that is needed to kill cancer.
A toddler in Florida has spurred a hunt for compatible blood donors, and the requirements are incredibly rigid.
"It's a humble request and I request it from my heart", Mughal said in the video. "My daughter's life very much depends on the blood, so please donate the blood for my daughter".