Immunogen sequence: LSSYNTIQSV FCCCCCCSVQ KRQMRTQISL STDEELPEKY TQRRRPWLSQ LSNKKQSNTG RVQPSKRKPL PPLPPSEVAE EKIQVKALYD FLP Antigen sequence identity: Mouse 72%; Rat 63%. TXK, along with the close relatives BMS, ITK, and BTK, signal downstream of antigen